I'd venture to guess that the level of automation in phone and car manufacturing is comparable.

Humans are cheaper than robots for things the size of a phone. You would need a lot of humans to pick up and move a unibody car frame so the economics and ergonomics don't work out the same.

They can be, and have been. Motorola and Nokia had very automated production facilities.[1] Even Foxconn in Shentzen is converting to automated assembly.

Iphones were not originally designed for easy automated assembly; early ones had internal discrete wires which had to be soldered. Newer ones use custom flexible printed wiring, which is more suited to automated assembly.

[1] https://www.youtube.com/watch?v=p-fLzrTVQjg

Phones are physically smaller, more dense, and tightly toleranced.