Because people are scared including myself even though I've distrusted some users.
Excluding someone from your Trust list often ends up in a retaliatory exclusion.
Distrusting high ranked user when you're still growing is like killing yourself, instead of getting backup from other high ranked user, they might choose to silent too because they're scared.
This forum is a scary place as a Newbie. Once you're established enough it's easy to stop caring, but that takes years.