I think it's down to when major shifts in gaming happen. The old stuff needs to feel different.
I'm not sure we've moved on enough from GTA3 to have left anything since then in the 'retro' category. And that's a seriously long time ago!
It was different in the early 2000s. It was less than a decade since consoles were almost entirely 2d (nod to Starfox etc), so the 16bit consoles felt a world away despite the relative short span of time.
I do think PS1 era stuff is retro now though. Character 3d platformers, tank-controlled adventure and horror games etc.