Well we're not talking about "how would you generally build an RBY team?"
We're talking about "what are the Pokemon that define the metagame in such a way that almost all bog standard teams would look almost exactly like this?"
Sure, there are substitutes for slots on your team based on what you need, but how often do you see someone using Persian over Tauros or Hypno over Alakazam? They're certainly viable options (if you can build a team correctly to incorporate them), but not as often used as their counterparts.
So yeah, you'll definitely see a lot of Tauros, Chansey, Exeggutor, and Golem/Rhydon in conjunction with each other. The general roles of these Pokemon are representatives of what goes on most standard teams, even if they themselves aren't being used.