I can understand where you are coming from. The thing is though, these assessments are exams. You don't go to any exam without preparing the best you possibly can (certainly not exams you seriously want to pass anyway), this includes mock exams for A levels etc. They do it because it works. A prep sim is essentially a mock exam. I can't talk for BA, but I have been to 3 airline open days where the pilot speakers actively encouraged doing it and made strong arguments for them.
I don't like them entirely because I think lots of them price gouge, but that's mostly down to a lack of serious competition in that particular market and is an entirely different conversation.