Google has already done something to make this possible with Android one devices, but apart from Nokia, almost all manufacturers have chosen to develop their own Android versions based on the source code.
So Android has become a container concept, and you also see Android versions without Google services on the devices on the market.
Then you have the consent authorities and consent agreements with telecom parties like KPN, Vodafone, T-Mobile, Orange, etc.

Google has already made the necessary changes to the Android design and is trying to make it mandatory for manufacturers that provide Google services to implement it, but then it remains the responsibility of the manufacturer to implement it properly.

And the manufacturer still has to test all those patches, against their own Android tweaks, before they roll out, because their camera app that makes the special stuff possible shouldn’t be broken by a camera driver update because the whole internet is angry at yours. Do not update this driver, some tech geeks are mad at you.
In addition, testing this update takes time and money, and getting it approved takes time and money. But half of the internet users are searching for the best phone back for claiming that they have the fastest hardware.
And the other half buys an Android phone from Samsung or Google because their updates are fine. And it’s this latter behavior, along with laws and regulations, that really gets manufacturers moving, and you see that Oppo and Xiaomi now also offer certain update guarantees on their high-end devices, but these models aren’t really cheap.

By the way, I would still like to see the Windows model on mobile phones. You can buy the hardware as well as the operating system, for an additional 100 euros during the lifetime you can expect all kinds of updates via the operating system supplier. This custom rom goes back to a time when hardware was very limited and couldn’t handle it, so ROM had to be highly optimized, but now custom rom projects or Android GSI projects have proven that they are not really necessary anymore.