Self-improvement is antisocial
It seems like individual growth is deemed to be bad for community. Social norms evolved to forbid excellence. We diminish people's accomplishments, rather than celebrate and raise them up. We turn our communities into crab buckets. We cut down tall poppies.
It's frustrating and confusing, if your self-actualisation trigger is learning and excelling for its own sake. It feels personal, as if others are trying to keep you, personally, down. However, it is actually an impersonal reaction which sacrifices your good for the good of the group. It's because change is dangerous.
Keep your friends
Life success (fall in love, get married, get accepted to an elite university, get promoted, get a new job, get pregnant, get published) comes with new responsibilities, relationships, locations, and communities. A friend's success signals that soon they will spend less time with you, and you're at risk of losing them entirely. People celebrate a friend's success in public, but they grieve in private. They can't even express their grief, lest they be accused of jealousy.
The best way to keep your social circle and your loved ones is to make sure they never grow, or if they do, that they grow at the same pace as you do ("come on, when are you going to have a baby?") Resentment towards people who move out of poor communities and "make something of themselves" exists because people who are left behind feel abandoned.
Don't fall behind
The classic theory of tall poppy syndrome is that prestige is a zero sum game: for someone to rise, others must fall. Exceptional individuals take away things which other people require to fulfil their needs (employment, romantic attention, social esteem and respect).
But motivations are actually feelings. It's not that we hate others for going too far ahead; we experience dread when we feel ourselves falling too far behind, and we want to stop it by all means necessary. As the joke goes: "when running from a bear, you don't have to be fast, you just have to be faster than your friend." When someone outpaces you, you can suddenly smell the warm, stinky breath of a bear closing in.
Switching sides: when people cut you down, it feels like they are against you personally, but they aren't. They are trying to protect themselves from this gut-wrenching fear. You are just an accidental target.
People prefer to keep everyone in their social group at roughly the same level, because in that way nobody feels like they're about to get eaten. When someone does, everyone else says "oh, bad luck", because they purposefully created a situation in which it is not possible to blame any one factor for that person's downfall. People don't want to be made aware of the existential threats!
Fish want to swim in shoals of similar shoalmates, people want to live in communities of similar people.
And that's why we don't want anyone to be good at things.