I imagine, as pressures came from within and without the classroom, teachers felt pressure to continue to move forward. Maybe you didn't have enough time to spend with the slower students to bring them up to where the others in the class are. Maybe you felt that you needed to march onward, ever onward to even mention all the things your state curriculum mandate. Maybe a number of other things could come into play.
At some point, though, you get to a frustrating point and think, "What's the least you can 'get' and still be able to continue?"
Even if a student cannot get motivated or understand the background or whatever, at least he can be expected to memorize the times tables, plug in values to the quadratic formula, or use his calculator to solve an equation. And so we teach there.
Derek Bruff, quoting Mazur in a recent tweet reminded me, "A problem is when you know where you want to get but don't know how to get there. We usually assign just the opposite."
Of course, we'd love to tell our students to do what they can to build a bookshelf and let them figure out all the stuff they need to do it, find directions, and then start taking steps to make it happen. In reality, though, a large number of students will ask why we're wanting them to build it, another percentage will see the wide expanse of possibility in front of them and freak out not knowing where to start, and most of them will just ask you to tell them how to do it. So, we let them fumble for a bit, but eventually, we get to the point where we say, "Ok, if you can't do it on your own, here is box with all the pieces you'll need and here are some instructions."
We tell ourselves that the lowest common denominator should at least be able to put the pieces together when we hand them exactly the correct parts they'll need and even give them a map for how to put them together. Even if they don't understand the Swedish words, they can look at the pictures and put slot A into tab B. And if you can't even do that, then I don't know how to help you.
Plus, if students can't think on their own, they will likely end up in jobs where they are meant to just follow orders from their boss. So, we're teaching them the life skills they'll need for that level of job.
This turns our classrooms into an Idiocracy. The students get so accustomed to being handed all the pieces and the map that they don't know how to think about problems when they are left to figure it out on their own. In some cases, the teacher may not even remember how to grow plants without using Brawndo.