But why do the programmers work for free on projects then? Why only the artists gets paid for just making a couple of models, when a programmer should know much more to code a game?
If you are a great artist, it's because you've spent hours creating art; you know a lot about perspective, form, balance, colour theory; you know how to use many different tools be they pens, pencils, charcoal, paints, photoshop, maya, etc. What matters most is experience, and the drive to hone their talents.
There are lots of hack artists though, who can maybe draw only certain types of objects in one style - just as there are lots of hack programmers who can barely put together a tetris clone with code they stole from google.
The reality is, artists AND programmers both get paid. These jobs exist, and people do them - talented, driven, practiced experienced people who never stop perfecting their craft - that's why it's hard to find good ones for free.