because as a gambler the hardest and most painful experience is when you see that you lost all that you have won trying to meet the conditions of the casino, because of the usage of bonus code and the requirements that comes with it.
This is exactly why casinos have this requirement coupled with almost all the bonuses they offer. They set high wagering requirement to be sure you will be at lost before meeting the requirement and being able to withdraw. You can't blame them, though. They run a business and we all know they don't give free money.
The only benifit for users is that bonuses money allow them to play more and extend a bit their gambling seessio.