Agree, I've learned to used "Turn-Based mode" when trying to steal, but it shouldn't be required if you passed the skill check.
Well...I could accept that, sometimes, and after a little while, an NPC realizes that something is missing even though they didn't sense they were getting robbed/pick-pocketed. That is an actually interesting thing to add, it's more realistic and adds consequences to actions.
But not so immediately and not systematically.
Also...again, "Turn-Based mode" is a bit annoying to trigger outside of combat (it's slow to turn it on and off), so it's rarely enjoyable/practical to use it.
Ah dang. Yeah. TB mode. That's probably what Larian is thinking. They're probably like, "It's the same with Stealth as with Perception checks. If you're moving too fast, it doesn't matter if you spot a trap, aka succeed in your Perception check. You are considered to have spotted the trap too late. Maybe you should try moving slower through an area, maybe in Stealth mode so you can spot traps in time to not set them off. Same with stealing outside of TB mode. You're trying to rush the pickpocketing. It's no wonder that they spot you."
The problem with that line of thinking is that it's incredibly annoying to players who don't realize what rules the DM is playing by. Maybe if they had some tool tips to say, "If you want to steal effectively, you need to use TB mode," that at least would let players know that if they don't use TB, they have a greater risk of getting caught.
Regardless, stealth in this game sucks. It needs an overhaul on DO many levels.