In short, because you are also paying for all of the un-paid hours those artists spent researching, studying, practicing and honing their skills. For your average professional, that's thousands of hours. That's a lot of non-paid preparation time that needs to be made up for. That, and of course freelancers have to compensate for their financial insecurity and lack of employment benefits (think of a pension, sick leave, health insurance and paid holidays). This is, in a nutshell, why professional freelance artists are so expensive.
I'm just clearing this up for those people who ask for "pro-quality art" and are then surprised that they will need more than $20 to afford it.
It is also for those beginning freelancers who undersell themselves dramatically because they feel they should adapt to their clients' unwillingness to pay for their hard work. I know, I'm actually guilty of having done so myself and yes I lost the majority of the clients I had by doubling my prices, but I also