In my case, my company 401k is with Fidelity, is still there as I just recently retired, and we are allowed to leave it there after retirement. However, they have revised funds in the last year or two. My target funds are Vanguard's (through Fidelity) that have an expense ratio of .05%, and I have some $ in an equity index fund with only a .02% expense ratio.
Other than the target funds available as part of the company 401k options there are not that many other options, and all but 2 have an expense ratio from .41% to .44% (yes, no zero after the decimal, $4.10 to $4.40 per $1,000). So that is why I am thinking of leaving. However, I have not yet talked to a Fidelity rep to see what options they have now that I'm retired, and I would still want to talk to a Vanguard rep to see what they have to offer.
To be honest, I wasn't planning on retiring at that time, so hadn't started any advance planning, but as often happens, when presented with the option, you take it and run, and sort it out later.