Why are ex-MBB consultants so bad?

Have seen a recurring theme with new analysts/associates who come from MBBs—They all suck. I can tell that they barely touched excel at their previous role, don’t understand what we are doing half of the time, and have an overwhelming arrogance factor that is unacceptable with the level of low quality work they’ve been producing. I also expect better PPT skills from them but those are mediocre at best. What exactly are MBBs teaching associates/biz analysts at the junior level? How to be the biggest asshole while simultaneously sucking at your job? Worst part is they aren’t improving. Really tainted my perception of them going forward.