Why does the existence of opportunity costs seem so obvious to me but controversial to some?

I read articles left and right about how opportunity cost doesn't make sense, and I just don't get it. Can someone explain what the argument is? We all clearly face choices all the time in our lives. Every hour one spends in one activity, they cannot be in another. Every dollar one spends in one thing is one less dollar they can spend in another. This is the whole concept of budgeting.