That's because of the goal + objective of being there as well as the switching costs!
Many people join MBB to build thier resume + network and open other doors. If they're leaving, it's because they've been able to leverage the brand to do what they really want to do.
Alternatively, if their goal is to rise up at the firm, it's best to stay and rise up the ladder to Partner as that's all relationships and longevity at the firm.
If you switch, you're either not meeting your goals in #1 or you're delaying your goals in #2.
The only real reason you would switch is if you're sure you want to stay a long time in strategy consulting, but you've just realize the culture match isn't there!