Android updates often do not come with kernel updates. Major kernel updates are difficult because of the tangle of proprietary modules in most Android kernels.
To partially answer GP's question:
Unofficial major kernel updates have happened in the past. OnePlus 6T, for example, shipped with 4.9.x, but supports mainline on pmOS.
To partially answer GP's question:
Unofficial major kernel updates have happened in the past. OnePlus 6T, for example, shipped with 4.9.x, but supports mainline on pmOS.