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