It's also about hardware abstraction. You don't want to rewrite your network stack for each platform but the unikernel can handle that.

It doesn't have to be unikernel either, Linux without anything else, your code being PID 1 is also good: no cruft and extra services that can be security holes, but great platform support, optimized network stack etc.

Of course if you just run on one hardware why not, just write the drivers you need.

How would something like this support logging, telemetry, debugging, etc as part of enterprise-grade software clusters if it has “no other services”?

It manages the services itself. Your app is one level down the totem pole. The http bit is just another service managed by your app.

Yeah an OS seems redundant here if you can just write freestanding bins.