Even if you had a replacement chip that was identical in form, fit, function, and reliability to the original (and you rarely get all of these), it would still have to be qualified as such. And that involves a non-trivial amount of engineering and testing plus any required government certification. As long as the currently designed-in chip is available at a satisfactory price it's difficult to justify the time and expense of replacing it in an existing product.
Plus I'm pretty sure that car manufacturers are already re-evaluating/modifying their chip choices for car models still in the design pipeline. The big question for them now is whether it makes sense to design in and test replacement chips just for the 2021 models. Designing-in, testing, and sourcing the new chips takes time and if they don't think they can complete this process before they're already into 2022 model production, then it makes sense to just wait until the current chips are available for whatever remaining 2021 models are scheduled to be produced.