You know how Ken Thompson and Dennis Ritchie created Unix on a PDP-7 in 1969? Well around 1971 they upgraded to a PDP-11 with a pair of RK05 disk packs (1.5 megabytes each) for storage.

The /bin vs /usr/bin split (and all the others) is an artifact of this, a 1970's implementation detail that got carried forward for decades by bureaucrats who never question why they're doing things. It stopped making any sense before Linux was ever invented, for multiple reasons: