It's been a few years since I interviewed there (for residency or fellowship) but here's my $0.02
When I interviewed I asked the same question. One of the R3s told me he only really knew about half of his (admittedly huge) class. Part of this was due to the 3 separate tracks at the time (PC, Boise and Cat - not sure what the setup is currently) and partly due to the fact that, even if you never leave Seattle, you spend time at 3 different, dramatically different and geographically separated sites. So it's best to think of UW not as one huge IM program but 5 or 6 different little ones. You'll no doubt find camaraderie and you'll have access to program leadership, but you'll have to work at it more than some other places.
FWIW, the 3 spread out hospital system was a huge downside for me and led to UW going from #1 on my pre-season list to #5.
UW/Harbor/PSVMC/SCCA is an enormous medical and research enterprise with hundreds of labs and clinicians. If you can't find a research or clinical mentor there, that's on you, not the program. People who want to go into fellowship and/or academic/research careers will have absolutely no trouble doing so coming out of UW.
They choose to stay there. Why would they leave? When I interviewed there for fellowship, the list of places that the current first year fellows had interviewed was ridonkulous.