Fear of the unknown. Those who have yet to go out always fear if they will hate their new job/progression. Which is pretty irrational when you realize that it is not that hard to get back into Big4 since high turnover is a perpetual issue.

In Big4, you know exactly what will be happening in your first 8-9 years and the nature of work. You will be getting promotion to SM if nothing wrong happens, and it is a matter of choice of clients. If you are unhappy with assurance, you can try to request to go to other functions (tax, advisory, others) which they are more flexible about it nowadays.

I have friends who are in audit asking me if I am bored in my new work the first 1-2 years after I left. I can't describe to them how happy I am to be outside.