[nonameiguess]

This isn't true. All hardware is swappable between boots. I've moved a root disk from one case to an entirely new case before, replacing literally every hardware component except the root volume but keeping the same OS. Not only is enumeration requires at boot, but event handlers for hot-swapping need to be available at all times. Users expect to be able to plug in and out USB devices, network cables, monitors, keyboards, and have it all just work.

Presumably, most faster boot OSes simply don't support as many devices. Amazon was able to get Firecracker to boot Linux damn near instantly by pretty much removing all hardware support from the kernel since they know for sure it's only ever running as a VM on a hypervisor they strictly control and won't change.

If you know for sure the devices you need, you can compile the kernel yourself and remove all the stuff you don't need and possibly even get rid of udev and just hardcode what it normally detects.