Alright let's try to come up with something here. We can all agree that succesful thievery cannot be most dependent on steal%. This will just lead to frustration of victims and endless cycle of nerf'n'buff.
We need more factors to be taken into account and be utilized by players. These would be (I took them from previous suggestions, don't be offended)
Darkness
Target's line of sight (better chances from behind)
Ammount of people at the crimescene
Tagging thieves or somehow paying more attention on certain people
But even then the little something is missing. It's about the final moment when you either feel someone lifting your pockets or don't. It can't be skill% roll vs perception because that will just encourage maxing steal and low cha players getting mad.
Okay I though about it for a while here it is: to succesfully steal something a thief has to remain certain ammount of time beyond his targets line of sight while observing the victim. After that his "intel bar" filled, he knows what to steal and he gets to advance towards his victim while still unnoticed, advance right behind him and then he can steal stuff.
Whether you're noticed by other people around is dependent on their ammount, lines of sight and darkness.
It's purely dependent on player's self awerness. If you're dumb and don't look around frequently, there is a chance that someone lifts you pockets.