why are decent chess players stuck at 450-500ish?


It doesn't look like you are stuck. Your rating has gone from 250 to near 500 in the last six months.

That's my question as well, I perfectly remember that I have played at least 7 games that have a great opening and middlegame and endgame, and yet stuck at 500-600 elo, I think these regions are the strongest

Too many blunders. Playing too fast.
True.
This is the OP's last lost game:
Black attacks the f2 pawn, white ignores it, and black ignores it. Now white attacks f7, and black ignores it.
At the end of the game, black is winning, throws it away, white gives it back, and now black (the OP) resigns in a winning position. WTF.
Can't really put more blunders into such a short game.
If you need help, please contact our Help and Support team.