Because in an all-in preflop scenario both players are sure to see all 5 cards and realise their true equity, and unless you have their top card dominated with a pocket pair like you have TT against their T8 or AA against their AK, you are not as big a favourite to win, as you might think.
For instance 6d5s, which you probably consider a "trash hand", will win 38,27% of the time against AhKc or basically 2 out of 5. Even if you have AhAc the same 6d5s will still take the win 19,21% of the time or basically 1 out of 5.
As others have said players with big stacks are not more likely to win an all-in preflop, but they are more likely to make a loose call especially if they are bad players. This is because bad players tend to feel a rush, when they get lucky and build up a big stack, and this makes them want to gamble even more.
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