>>csmant+(OP)
This is what happens when a system is designed by multiple people and companies over a long period of time. An amalgam of ideas which are there just because. There's no reason Linux should be like this. e.g., see https://gobolinux.org/ which has more sane dirs.
>>behnam+Cm
Linux does not use this split any more. Many of these dirs were merged back together. The "/usr merge" was adopted by Debian, Ubuntu, Fedora, Red Hat, Arch Linux, openSUSE and other major distros: