Why Mac OS X requires any amount of free disk space? I understand the OS needs some free disk space for running. So does windows or any *nix.

But OS X is regarded as a very polished system and my question here is more like, shouldn't it already reserve all disk space that it needs so it won't ever complain (or bug out) about not having enough space?

If it's 10 gb, then get a 64 gb disk and say "you got 44 gb free space, because 20 is used by the system" (supposing OSX already takes 10). Just take the free space needed already! Make a partition if need to be! Hide it if you really want.

Then it wouldn't need to bother the user with worrying about that. There must be a good reason why it's like this, but I couldn't find out. Can you please explain?
The amount of space the OS needs is neither fixed nor predictable. It depends on a huge number of factors: what programs you run, what you do in them, what background activity they trigger, what background activity happens for other reasons, etc. Hard drive space is used for overflow from RAM, logs, temporary files, etc.  If the system reserved a fixed amount of space for this, it would wind up being bigger than it needs to be a lot of the time (and hence waste space), and too small some of the time (which would be very bad).