Because those who answers questions puts in more effort. As simple as that. Like if you ask a question you are the one that's wanting solutions to problems. And you are asking and seeking help. Now isn't it a bit absurd that the one that is asking for help should get something in return too?

But Ive seen some decent amount of rewards on some questions in musing. If you see the top answers catagory you'll find many wiith decent amount of upvotes. Now if you ask, why is that? That's because question that gets less upvoted is thought of having lesser value. And those are very few in amounts. For example, look at the question you asked. Does it bring enough value to get a monstrous upvote? Answer that and you'll find your answer.