These conversions are super common. What am I missing?

That is done to kill the "chop shops" criminals used to steal cars and then break them up for parts. You can't do that with computers because of that pairing and so stealing a car pays much less and in turn is less common. (it still happens, it just doesn't pay as well)

The trick is to pick up the main computer and the paired drive unit(s) by picking up a whole vehicle (with a salvage title). There are shops in LA, and elsewhere who do conversions this way.