The short version is it's because everything about you is your fault and responsibility. If you're not smart, you're just not trying hard enough. If you're poor, you're just lazy. This creates a recipe for absolutely crushing children if you treat them harshly -- it's not just a matter of "You didn't do well, but that's okay.", it's become "You didn't do well, and that's your own fault.". It's not stated overtly like that, but the sentiment is so pervasive and has been so deeply ingrained in people over the past thirty years that it doesn't have to be said directly.
This ends up creating a lot of weird contradictions in education environments. If you want to be kind to someone, which is a lot of peoples' instinct when dealing with children, you have a very hard time with criticizing their ideas or capabilities because you're no longer just criticizing their ideas or capabilities -- you're implicitly criticizing the person directly. From the other direction, education has become a push for everyone to succeed... Because if they fail, you've failed to get them to work hard enough. It's not that anything is beyond anyone's capabilities -- it's that it's either their personal failure for failing, or your personal failure for not pushing them to work hard enough. Standardized testing is the poster child of this latter issue.
Of course, this makes for a lot of problems later in life, too, when someone who's been told their entire life that they can do anything if they want it enough... Fails to do that anything. They obviously just didn't want it hard enough, or didn't work hard enough. Ending up in a mediocre (or worse) job is their own personal failing, and people often need to find some way to escape from that (be it escapism like video games, TV, books, or movies, or mind-altering substances, or spiraling down into depression). Doesn't make for a great society at all.
This isn't the only problem with the world or society, but it's a really big one and, for most people, really difficult to even notice.