If Graphene can do it, why can't they?

They can, they just don't want to add more engineering hours to that I imagine

Grapheneos is, to my understanding, a dedicated development effort that specifically targets pixels.

Lineageos on the otherhand, provides scaffolding that volunteers can use to support phones of their choosing. It's very best effort and ymmv between phones. A universal upgrade option is just asking for trouble as it's not guaranteed all phones will behave the same/well.

FWIW in my experience upgrading Android versions works, mostly, as long as you remember to uninstall the old Google Play services and then install the new ones.

However, without a tested migration path, it may break your phone and make you factory reset + reflash the ROM if it doesn't work out, and there's nobody you can turn to or blame when that goes wrong. There's no official support, but that doesn't mean it'll never work.

Testing migration paths is a massive pain, especially when you're upgrading a whole bunch of parts all at once, and volunteers have more fun and frankly more important things to work on.