Could someone please explain to me how students taking AP classes has become so mainstream in today's world?
At least part of the credit (or blame, depending on your outlook) goes to Jay Mathews at the Washington Post and his ranking of high schools based on the number of AP and/or IB courses taken on average by each graduating senior over four years of high school. The WaPo ranking of high schools started in 1998.
It's not necessary to take AP's to get into a "decent" college, where the definition of "decent" is going to vary from student to student.
The problem is if a student wants to apply to a highly selective college which expects that applicants have taken the most rigorous academic courses possible at their high school. If the student went to a school with no APs, no IB, but took the toughest courses offered, that's different than going to a school that offers a couple of dozen AP courses, but the student only takes one or two.