Parts are made to only fit in the correct spot with registration points (holes, edges, posts) that need to match the jig they are loaded into. Even home 3D printer servos easily position the tool heads to within 0.1mm.

Furthermore, the spot welds do not need sub-millimeter precision. The spot welders were once guided by hand and placed randomly along the panel joints.

servos are very precise, that's why get get used for this sort of operation. they might need to be recalibrated as the parts inside them or attatched to them get physically worn down, but that's the sort of thing that would happen during a maintenance shutdown, not inbetween each workpiece.

Why would you not want basic feedback loop built into a control system so it is not high maintenance?

Adding a say simple laser distance measurement sensor to make sure the piece is at the distance it is supposed to be is not difficult, computer vision and AI maybe overkill but a simple sensor ?

Also a lot of inspection seems to done by hand, while good inspectors can be smart about finding issues , Why not scan the entire car and see if there are abnormalities over accepted range ? it is like manual QA, I rather have automation suite rather that inconsistency of a manual process.

I am sure people are already thinking about it, Musk certainly was making a big push for more automation (and ran into a ton of difficulties )

Some of them compensate for wear if the wear rate is known (eg: 1000 welds cause .5mm erosion on the electrodes). You can only do this until the tool geometry becomes unsuitable for that particular operation.