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1. Creating samples, just like anything else, is time consuming and takes an effort. And time, as we know = money. Everyone, not just you, whould like free samples and together it equals to weeks and months of unpaid labor. 

2. Creative ideas and suggestions are also a product. When you pay for creative work, you pay not only for the final product, but also for hours spent researching, checking out your competition, brainstorming, sketching, years of experience (and talent), consultations, emails, all the designs that were made before the final design (including those that haven´t been used), fixing and editing stuff, final touches... This works in other fields too. When a movie is shot, there is always more footage than what you see on screen. And yet, every actor and crew member is payed for all the hours spent shooting, not just for the final 60 minutes.

3. In many cases, I am not the only graphic designer, who is asked for a free "sample". It is common, that a company will ask 20 different designers (or even more) for a free "sample". If each of them works just 5 hours, a company can be 100% sure of getting at least 20 different samples equal to 100 h of free labor. If all of those designers charge 10 €/h, it makes 1000 €. And that is a full monthly salary for some graphic designers. However, every graphic designer in the example above has only 1:20 chance, that his/her free "sample" will bring him/her one time payment (usually in the amount much smaller than 1000 €, since his/her work share was "only" 5 hours).

4. Would you do spec work or offer free samples? If you whould, it would most likely be a planned marketing strategy, such as a moisturizer sample in a magazine or a cheese sampling promotion in a supermarket. This marketing strategy was your idea, it was planned in advance, certain amount of money was set aside for it (and counted towards loses), calculations have been made and it was concluded, that this promotion will most likely bring increase in sales of cheese or moisturizer. But the customer himself didn´t demand or ask for a free sample and there is a good change that he (or someone else) whould buy that cheese or moisturizer even without it, or using a different marketing strategy comming from you. On top of that, unlike cheese, which already has a recipe created and therefore it can be mass produced over and over again, creative work is usually unique every time for each customer. 

5. There is a risk of theft of creative idea or a free sample. Followed by "ghosting" (ignoring emails and phone calls) from a client, after a graphic designer already spent hours of working to provide that free sample.

6. Time frame, that is given to provide a "free sample" usually forces a designer to choose between a quality of delivered work and finishing it before he/she looses too much time and money on it. 

7. Risk of scope creeps using the "quick" and "free" sample option to ask for additional amount of work , tasks, variations and modifications, which were not part of the deal at first. This can very easily extend the original 4 hours of free work to 8, 12, 20... if a graphic designer doesn´t "cut off" a scope creep client.

8. Just because something looks simple, it doesn´t mean it was done quickly. It took more than 17 hours to design a Nike logo, which is "just" a "swoosh". If you want to know why that is, read a point number 2 again. In other words, just because you tell a designer, that you want "something quick and simple", doesn´t guarantee, that it will be indeed created as quickly and simply as you assume. To create "simple" things in a way that looks amazing (and is UX friendly) takes time.

9. Companies asking for free samples often don´t give enough necessary information. If you say that you will "leave it up to a graphic designer" or "create something for me, then we shall see", a designer doesn´t have anything to work with. His/her work on your project is just a guessing game ar a gamble, during which they try to figure out your preference or personal taste. This increases a risk of spending time creating something, that you don´t like. It puts a designer into a financial loss and wastes his/her time. If a client sees a potential in designer´s skills (but they don´t like their first sample), they will usually ask for another "sample" (still for free?). This is directly connected to a problem number 10.

10. Customer asking for free samples from 20 different graphic designers usually doesn´t have time (or don´t want to put it aside) for individual consultations. If they split their time equally between all 20 designers, it wouldn´t be so financially beneficial for them, since time is money. In an attempt to save as much time as possible, and therefore also money, they often reduce their communication to a short vague assignment (just few lines of text), which is often not enough to create quality product.

11. Every paying client pays the price for speculations of non-paying clients (at least partially).  if enough companies ask a graphic designer for free samples on a regular bases, his/her hourly rate will eventually increase to make up for the lost time spent creating these free samples in an effort to minimize financial losses. 

12. Project in progress and sketches can look very much different from the final work. It is very common to ask for "quick" or "paper napkin" sketches, but it isn´t always a good idea. In its basic form a brainstorming and sketching stage has the most value for the artist himself/herself. This often includes only simple shapes, basic poses (sometimes just stick figures), collages... Graphic designers know, what they mean by that and how it helps further creative process. They have a clear plan (or a vision) in their head, and therefore don´t need more detailed sketches at this stage. However, to a bystander, who lacks context, these initial sketches don´t tell much on their own (at least not without explanation). A designer will then spend more time (often extra hours) improving these ideas to make them "presentable" for a client. And at that point it is actually no longer sketching or quick collage stage, but rather a stage of creating first prototypes.

13. Lack of legal protection is unfair towards graphic designer (but also towards a client). Under a normal circumstances, an author is legally protected against stealing his/her creative work and ideas and using it without his/her permission. This protection is often lost (or very hard to claim) in a moment, when he/she agrees to create work without a prior written agreement (contract). Graphic designer in this case has nothing to lean on in case of any legal issues and a whole collaboration (and potential reward) is left on "good will" of a client.