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Why would one choose NASM over FASM? The makers of KolibriOS (an OS written entirely in assembler) chose FASM but the reason eludes me.

To me all assemblers look alike and it would even be feasible to write one's own just for the heck of it.

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I have not used FASM so I can't comment on it, but there are quite large differences between assemblers in general. Each is essentially a (non-optimizing) compiler for a different extension language. Writing your own assembler that literally just assembles is trivial, but writing raw assembly instruction by instruction is tedious and not especially practical. If you're serious about writing it, you'll want macros and other forms of metaprogramming; as a programmer, the entire point of your existence is to automate tedious, repetitive tasks, and better assemblers help with that. Programming your own assembler is a natural progression of metaprogramming if you stick with assembly long enough, but first you should learn with using an existing one so you can begin to understand your own workflow well enough to know what tools you want for reducing repetition and where the existing one falls short.

And why not? You can just use CALL and make subroutines for repetitive tasks. Why would one need macro functionality which is basically inlining code like in C/C++. Inlining also makes the executable needlessly large. It may have its uses but I suspect most of the time just calling a function or subroutine is more desirable.

Calls have overhead, so they shouldn't be used for everything, especially in hot paths. The entire point of writing assembly is to optimize performance. Even on very limited hardware, or rather especially on limited hardware, you'd usually much rather trade code size for performance.

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