AFAIK it's basically just that math education has been moving more advanced concepts earlier and earlier at least as an optional course. Like calculus has gone from a post-grad level thing to a normal college course to something people can elect to take in high school, algebra went from college level material to normal high school level stuff that people start getting introduced to in middle school, etc.

But also a lot of people struggle with the most basic things in high school, graduate, then forget all of the incomplete-understanding they had by the time they have kids. It's very likely that adults who are baffled and enraged at seeing some basic algebra problem also struggled with math in school and are embarrassed and frustrated that their skills have only gotten worse since then, on top of the possibility that they never even got to algebra when they were in school and instead went through the remedial math track that maybe reaches basic pre-algebra material in their senior year.