A big issue here is that GameFreak didn’t even know about the Ditto is a failed Mew clone theory despite making a lot more sense and was much more popular.It probably wasn't to begin with*, but there's no way Game Freak isn't aware of it by now (even if they might pretend otherwise) so they probably are leaning into it at least a little.
You should also consider the internals of GameFreak and TPCi.
Besides Pokemon being designed by different people, including entire lines being designed by different people, you also have to consider the marketing of Pokemon as well.
Having it so there can only be 12 Fire Starters and all those Fire Starters have predictable designs is not a good marketing strategy. The Chinese Zodiac would definitely be prevented by executives who understand how bad of an idea it is.