I dug into this a while ago; I recall (but can't source at the moment) an eventual statement by the DO guys to the effect of:
1. their original "inject kernel into OS and boot from it that way" architecture was designed that way on purpose, so that users could downgrade kernels out-of-band as a way to rescue a bad upgrade;
2. but now they've got thousands of droplets configured that way, so switching architectures to something sensible will require either A. forcibly installing grub on, and restarting, every one of their nodes (not something people expect out of a VPS provider), or B. coming up with some sort of glue that can manage both the nodes expecting an injected kernel, and nodes that want to boot on their own, and providing a way to transition your nodes from one to the other.
Basically, it's a mess. A bit like the problems Heroku had with its Alpine stack.
1. their original "inject kernel into OS and boot from it that way" architecture was designed that way on purpose, so that users could downgrade kernels out-of-band as a way to rescue a bad upgrade;
2. but now they've got thousands of droplets configured that way, so switching architectures to something sensible will require either A. forcibly installing grub on, and restarting, every one of their nodes (not something people expect out of a VPS provider), or B. coming up with some sort of glue that can manage both the nodes expecting an injected kernel, and nodes that want to boot on their own, and providing a way to transition your nodes from one to the other.
Basically, it's a mess. A bit like the problems Heroku had with its Alpine stack.