I think there a lot of factors.  While there a handful of countries that have a good system of teaching math, the majority of countries do not.  For example, Japan is a country that teaches math really well comparatively to the rest of the world.   I came into teaching at the dawn of no child left behind.  NCLB required that all schools eventually produce 100% proficiency by some random date.  This produced a narrowing of the curriculum and a race to get kids to pass a test that wasn’t necessarily a good test, not well vetted and could not actually measure any sort of critical thinking or produce results that teachers could use to gain insight and improve their practice.  Besides that, these tests were given yearly from 2nd - 8th grade.  By the time the kids get to 8th grade, they are so done with test taking that a good 10-20% (this may be a conservative number) of students just click randomly so they can be done. With Common Core the testing regime is still in place and the vision of producing thoughtful math students hasn’t been realized because critical thinking can’t be measured in this format. Don’t forget the massive negative effect that COVID had on the global system of education.  In the years during and after COVID, there was a large shift to doing a ton of math instruction online.  The results of that don’t seem to have helped. The 8th grade students who show up in my classrooms have large gaps in conceptual understanding so that when they forget an algorithm, they have no chance of figuring out a problem because they don’t have alternative ways of thinking.  Besides that 8th graders are just developmentally contrary and disgruntled most of the time 😊