Well, typically a car manufacturer doesn't make a lot of the ancillary components that are recalled.
They outsource them to a third party who also likely has a multitude of other customers.
So first, they have to identify through testing WHAT the actual problem is. Then engineer, design, and validate a solution (often times in conjunction with the manufacturer of the failed component). Then prototype the solution and test the fix. Then cut a PO to the outsource manufacturer to start production. If that mfg purchases manufactured assemblies that go into theirs, they have to repeat the steps above before they get that component. The mfg has to source materials, schedule time onto the production lines, ship, etc...
It all takes time and the more complex, the longer it takes.
The reality of today is that we the consumer want more and more creature comforts. Those add complexity. Then we want it cheap, and we want it fast. Then when it breaks we want it fixed immediately. There is a saying in manufacturing. If you want it to be good, it won't be cheap nor fast. If you want it cheap and fast, it won't be good.