Flask is too minimal. Using DRF is like having code written for you. Because, you know, the code was written (and battle-tested) for you. Why start from a log every time when you can get a wheel for free?

DRF includes all the serialization and loose object structure for you to build off of.

We've been using DRF pretty seriously for over a year and have yet to find a feature that we want that isn't already implemented or easy to implement on top of it.