One of the problems is that a candidate might sit on an offer for a long time. The best candidates want to wait until the interview season is over to evaluate all their offers. On the other hand, some schools give one week deadlines on offers but any good candidate will turn those down.

It is hard on both sides. The entire process was about 6 months when I went through it, and I had offers expiring before I even finished all the onsites.

Disclaimer: I’m a CS prof at a R1 university in my first year.