These things are possible with APIs like CreateProcess or posix_spawn, but involve the API ending up with a million little customizable parameters in the attemp

These things are possible with APIs like CreateProcess or posix_spawn, but involve the API ending up with a million little customizable parameters in the attempt to simulate all of the behaviors that can trivially be implemented if you just support "let me run some code in the target process before I replace its memory"... which is like, exactly why "easier to implement" sort of means: something is often easier to implement if it is a single universal and general primitive that forms an elegant solution to a wide set of problems rather than being a myriad collection of hacks for various use cases. As an API--particularly on systems with copy-on-write pages--fork is amazing.