There's a lot more to Yocto than just building the kernel. It's still useful when kernel support is upstreamed, such as including vendor tooling and test programs.

Upstreaming also takes a very long time and is usually incomplete. Even when some upstream support is available you will often have to use the vendor specific kernel if you want to use certain features of the chip.

Nobody can wait around for upstream support for everything. It takes far too long and likely won't ever cover every feature of a modern chip.