It's more efficient to let the unikernel start a HTTP server and keep running for a few minutes. With AWS Lambda you're paying for walltime even if the CPU is idle because it makes a long running network request. Imagine you're making 10 HTTP requests per second and each has a very long latency of 5 seconds. With 10 AWS Lambda "instances" running concurrently you're billed for 50 seconds. With a single EC2 instance you're billed for 5 seconds.
when the OS is just some library functions statically compiled in and the entire binary is a few megabytes that you can load directly into memory, why not?
The point being that docker is actually a really clunky thing that is reinventing the virtual server ecosystem within the OS which is pointless.
Serverless systems are better implemented as unikernels than docker images.
Unikernels make it possible to boot an entire OS in milliseconds, service an inbound web request, then disappear.