Sure, unikernels are a thing.
But being able to run a full OS is convenient, and often still faster overall as there are magnitudes more people working at optimizing general purpose OSs than specialized unikernels.
By the time you got done writing device drivers, etc. you would just have another OS.
Also “without OS overhead” - things sitting in memory don’t necessary detract from performance, but not having hardware acceleration for lack of drivers certainly can.
The only overhead guaranteed during execution by the OS is context switching, and if you want to avoid that you’re basically writing an RTOS for an embedded system.
Why do we even waste resources on OS when we can write e.g UEFI application and run it "directly" and let it manage resources without OS overhead
Nowadays runtimes like Java's, etc, etc. are doing more and more tasks that previously were managed by OSes