Unless they're assuming it's exploitable on Apple Silicon as well, or are being extra careful just in case.
Things like OpenCore Legacy Patcher prove it's possible; they just don't want to.
I don't think anyone feels entitled to new features in perpetuity. Security updates only would be fine thank you.
Don't tell me the richest company in the world can't pay for a couple of developers who just want to rest and vest to take care of and test the legacy platforms. A cushy job and you keep the customers happy.
Ironically the best way to stay safe on these computers is to install Windows or Linux.
And it's far more than just a "couple of developers" to support older operating systems.
For example, I have 2015 macbook pro. The last macos release for it is Monterey. Even brew has problems with that, erroring out when installing packages like libpng and complaining, that I should upgrade xcode cli tools. Which I can't.