The process is somewhat archaic (often involving mailing around paper checks) and I imagine many people just don't want to deal. Rolling over pulls your money out of the market which means you could miss a good day (or a bad day).

I left a trail of 3-4 accounts until just recently, when I rolled them all over to my current Vanguard 401k. They were all invested in the same Vanguard fund so there's not much change other than simplicity.

With Vanguard at least, while you still have to get a paper check from your employer. But you can electronically deposit it into your IRA account by taking a picture of your check.