If a banned user creates a sock and devotes himself to
making useful edits,
without rekindling the fires of disruption, then how does it benefit the
project to tracking him down and kicking him off? That sort of thing merely
causes frustration, unpleasantness, and more disruption.
Which is why, as far as I know, it doesn't happen. It's always been
the case that a banned user can come back with a new account and as
long as no-one can tell they're the same person, there is no problem.
The only way someone could tell is by illegal checkusers - do you have
evidence of checkuser abuse?