[2021] Multiple accounts on console ok, but not on pc?

Me and my brother used to share a console copy of rainbow six siege on the PS4, but now that we moved on to a pc we are having trouble with linking or sharing the game between us, he bought it on steam, but the family sharing option simply tells me to "purchase" the game instead of the usual "borrow". Is sharing the game between accounts unavailable on PC? why, if it works just fine on console? im just confused and really not feel like buying a SECOND copy of the same game that we usually could share just fine