Dave "EEVblog" Jones did a review of a (then) $0.03 microcontroller (Padauk) a while back (sorry, I don't know the exact episode).
Iirc an important caveat was that it was a one-time programmable (OTP) part. So you buy a bunch of them, programming failure or firmware-under-test doesn't work? -> toss the part. Of course that isn't an issue for a $0.03 part. But it can be an issue in terms of a board you want it on. Either that means discarding (breakout) boards too, or for development you'd need some kind of adapter to put bare ICs in.
Such annoyances only make sense for high-volume, low cost applications. Which is eactly where parts like that go in.