A big part of the problem is the names we've chosen - Artificial Intelligence and Machine Learning. A more accurate, though less sexy, name would have been "Mathematical Pattern Recognition". We can do amazing things with classifiers but we shouldn't fool ourselves into thinking it represents "intelligence".

IMHO what's still lacking is the feedback loop.

Current systems are limited to ingesting input and providing output.

A big part of medical diagnosis, however, is to do follow-up exploration based on results of a previous examination.

This is a big part of "intelligence" that's still missing entirely from all approaches that I'm aware of, i.e. the ability to ask further questions/request data based on preliminary results from previous inputs.

Brains and intelligence are pretty much just pattern recognition as well - "neurons that fire together wire together"

Brains in isolation, yes.

But intelligence isn't just a "brain in a jar" situation. Intelligence requires interaction with the environment - you'd never be able to tell whether a Boltzmann Brain is intelligent from just observing it, for example.

Also because linear regression is a form of AI. (Anything done by a computer rather than a human is.)