This has it backwards - lambda is only suitable for the cloud equivalent of shell scripts, anything more complex and it becomes an unmaintainable rats nest that cannot be debugged.
No one has to, there are 1000s of alternatives but some people chose to. Don't ask me why.
I, in fact, use Elixir serverless: I just send functions to my BEAM and it runs them for me.
I know (think) you're half joking, but I wonder if you could abuse the hot-reload system to injected your function (assuming you don't want to eval strings) into some "lambda" module and run it.