Why bother videorecording interviews at all?

We don't record interviews. By "remote video interviews" I meant "remote interviews by videoconferencing such as Skype or Google Hangouts" which I assumed was what you meant when you proposed candidates use their own office or couch.

  why not just tell them ahead of time what their
  environment needs to do when they get there?

We did this, including a github test project they could check out and build to make sure everything was working. Still, about 50% of candidates arrived without a working environment.

We chose to supply a known-good laptop instead of rejecting such candidates instantly when they've spent time travelling to us etc.

  every time I hear someone explain how challenging or
  untenable our process is, I keep wondering, "what am
  I doing wrong to make this work so well for us?"

I'm not saying your process is challenging for _you_ given _your_ situation; I'm sure it works very well for you! I'm saying it's challenging for _us_ given _our_ situation :)