Why Arch dont have /sbin nor /usr/sbin directory differing to most Linux ?
/bin
/sbin
/usr/sbin
should all be links to /usr/bin
If you don't have the links it's a problem and you should reinstall the package 'filesystem'
If you do have the links https://www.freedesktop.org/wiki/Software/systemd/TheCaseForTheUsrMerge/ explains the rational.
Different distros do it differently (in some cases) because they are different distros and can.
Make sense to me. There was a time when the filesystem tree was really messy with X programs in their own /usr/X11R6/bin and sometimes configs in /usr/X11R6/etc. It was confusing to a new Linux user (me at least).
Other distros also often keep their libraries in different places.
Ah, yeah, the brilliant move to /usr/bin "to make / clutter-free", which resulted in /bin and /sbin being symlinks to /usr/bin, / just as cluttered as before and /usr/bin a helter-skelter of thousands of binaries. KISS, my ass.
Remove the symlinks and they'd be chaos as a million perl and bash scripts scream in unison.
I admit I was against the change when announced (Luddite is in my nature more often than not) but its grown on me over time.