Because phones are a mess of out of tree patches specific to that phone model with zero hope of being upstreamed into the Linux kernel without a cleaner rewrite because it’s not good, it’s made to work and nothing more.

It’s improved but companies like Qualcomm also used to basically drop the code to the manufacturers when the chip launches and then move on with little maintenance for the code and stop maintaining the code once the chip is not produced anymore. Manufacturers don’t have the expertise to maintain that forever nor the will, so you end up with a kernel that keeps aging and isn’t keeping up with Android and the community hasn’t been successful in integrating it all either.

Google’s been pushing hard for this to improve but they’re the only ones to even care. Samsung and others would much rather sell you a new phone.