Because it is a long read and normally, the majority of the new sign-ups don't really read them at all. Let's embrace the reality about that but when problems are encountered and we have to take a look if something has been violated, we go back there and carefully review it. And if someone decided just as what you've said not following bonuses or promos, there will be no problems anymore but where's the fun at? it can be found there.
While you are right, 99% of all people will always skip the terms and conditions and just blindly accept it. When a problems arises, people will often complain about it as that is human nature also. For bonuses and promotions there will always be certain rules that need to be followed but everyone needs to do their own due diligence before participating in it or accepting a bonus.