I've ridden a ton of 200k perms, and plenty of longer brevets. I've thought about riding some of the longer perms around, but never have.
I don't want to overanalyze, but I think it has to do with how much the ride impacts my weekend. A 200k is a day; I might get up a bit early, but I'll have a normal evening at home and Sunday is unaffected. A 100k is a morning. I can even do a PTP 200k up to Olympia or down to Eugene, Amtrak home, and it's a one-day impact. A 300k starts to creep into impacting Sunday, with sleeping late and just being tired. I like to do other stuff - hike with my wife, etc. Thus far, my choice has been if I'm going to knock out a weekend, it's going to be a brevet. Not to say I'd never ride a longer perm, but that's what I've done to date.
I probably just overanalyzed it.
I'm in the midst of designing a perm around Mt Hood. Starting from here (just outside Portland), it would be around 240k. I could just make it a long 200k, or add some miles and make it a 300k, or move the start/finish out and keep it to 200k. I'm tending to the the latter. When I ride it, I'd take transit to/from the start/finish. Why do that rather than make it a long 200 and ride - particularly since the elapsed time would be about the same? Shrug.
In my RWGPS account I have a number of draft perms, including 300k and 400k. It is a bit of work, and right now I'd be creating a route I probably wouldn't ride myself.