RE3 Zombies are pretty much redundant relatively since it is geared mostly towards Nemesis chasing the player; so they didn't feel the need to put as much detail as they put in with RE2 zombies
Re 2 is them having fun with the ai and zombies physics… re3 is more mainstream being lead down the triple A path seeing how upset people got they fixed re4 to be close to re 2 type of gameplay…. Even added mercenaries back but i fee we still missed re 2 having more single player content