because many times people want to be able to run new drivers (e.g. with new hardware support) on an LTS kernel.
lots of vendors support this, and most do it by maintaining a version of their driver that can work with a wide range of kernel versions that detect and adapt to what APIs get added/renamed/removed.
Citing being able to use an LTS kernel as the reason seems like circular logic. If every device had a mainlined driver, what would you need an LTS kernel for, rather than just always using the latest?
Why? We want as many drivers as possible to be in the kernel.