What does this even mean? It can do a lot for me, therefore it's bad?
We use DRF extensively, but we don't come close to using everything that it can do. We don't tend to use ViewSets or the auto-routing features, but the fact that it can do those things doesn't hurt us.
DRF gets so much attention from such a large community. It's constantly improving to the point where we don't have to worry about it. If you fall victim to NIH Syndrome and roll your own, you have to be proactive about finding and addressing your own issues, as well as keeping up with advancements around the ecosystem.
I'd just rather let the people who focus on DRF handle that, and we'll throw a pull request over the wall if we run into anything that hasn't been addressed.
So yeah, there are efficiency gains from DRF, I agree. But (in my opinion), the gains are far less in architectures with separate applications all using DRF. In that case, something like Flask makes more sense to me, purely for the benefit of debugging, logic clarity, and independence.
I recommend a great talk by Kenneth Reitz on exactly this topic. [1] (He's given a variation on it multiple times -- just search youtube.)
Django is pretty easy to slim down. Yank the middleware you don't need, stick to what you do. No need for stuff like context processors if you are using DRF. You don't have to use models or the ORM at all if you don't want.
I think the line count would be very similar for many cases. For example, if your flask app needed DB access, you'd end up pulling in and boilerplating an ORM anyway. Ditto for sessions/authentication/etc.