I can think of some things you probably wouldn't want to reimplement yourself. Video encode/decode, for one. Cryptography.

I would assume the console makers provide platform native APIs for that kind of functionality, though they might be relatively low level. This would seem to confirm Nintendo does for video decoding.

A developer COULD use the time they save by reusing code in order to optimize the end result and end up with a better product. They may choose not to.

I definitely agree that it's not necessarily the use of third-party engines or APIs that results in poor optimization. And it's also certainly true that going the DIY route does not necessarily mean a well-optimized game, as Game Freak has demonstrated on the Switch.