While this would be good for consumers, it's bad for the automakers' bottom line - parts are a significant source of highly profitable revenue with gross margins often in excess of 100%. Warranty work performed by dealers almost certainly mandates the use of OEM parts and insurers might insist as well for body work, thus there's some demand inelasticity. By the time the market for a given headlight assembly becomes price-sensitive the aftermarket can chase those low margins.
...
When I worked for a small manufacturer we marked up commodity stuff by 400% over
COGS - even more for our own proprietary components. Net margins on that line of business were significantly lower since processing orders requires most of an employee's time, required carrying inventory for long periods of time, and also necessitated ordering irregular inventory for the spares business
(being able to reliably ship spares within ~1 business day was critical to the overall business).
Why the independent service firms bought commodity parts from us was always somewhat mysterious - they could read the labels and order from the likes of Mouser, Grainger, et al easily enough. Some may have had service arrangements with their customers requiring the use of OEM parts; others may have had agreements billing replacement parts on a "cost plus" arrangement. Either way thus paying 4x as much for a microswitch meant 4x the margin so long as they had a magic OEM invoice.