Have you ever seen someone use a turn lane to only jump out of it at the last moment?

Yeah, when they have their turn signal on to indicate they want to change to a different lane

Leaving the turn signal on when you’re already where you want to be is the more confusing thing. I know most people do it because it’s taught that way in driving schools, but it’s a matter of habit, not actually logical if we’d design the system anew and everyone learned from scratch