I wouldn't say rare, rather "uncommon". RTS games were more common on the PSone than later systems. Every PSone RTS I own...3 C&C's, Dune 2000 and Warzone 2100 supports the PSone mouse. Strategy-RPG's are also fairly common.
MMO's also are not rare. I played my first MMO on the PS2 back in 2003, EQOA, RAM wasn't the issue you think it was. I also played Final Fantasy XI on the PS2. They are both traditional MMO's In fact, WoW is a LOT like EQOA in being an MMORPG for the Masses.
Final Fantasy XIV, DC Universe Online and FreeRealms on the PS3 are also traditional MMO's
And Final Fantasy XIV, Onigiri, DC Universe Online, The Elder Scrolls Online, Neverwinter and soon Trove and Star Trek Online, on the PS4. All "real MMO's".