I want to take a look at one additional reason why we don’t use complex bans - they often don’t actually accomplish the desired results.
We tried to limit weather in Gen 5 OU by restricting combinations of abilities. It is looked back upon as something that we should not have done, because it didn’t solve the problem in the metagame.
We tried to restrict the number of Pokémon that could use the move “Baton Pass.” It is looked back upon as something we should not have done, because it didn’t solve the problem in the metagame.
We tried in Gen 8 Ubers to restrict which Pokémon could Dynamax. It is looked back upon as something we shouldn’t have done, because it didn’t solve the problem in the metagame.
Complex bans don’t work. If we try to apply this to the current metagame, and write rules like “Regieleki should not be able to Tera,” we need to seriously reevaluate what we are trying to do: either Regieleki is too powerful in the current ruleset (in which case tiering action is needed), or that the current ruleset is creating an environment that is not hospitable to a good metagame (in which case, we should look at the ruleset).
Generally, if you were to do a complex ban, one of these would remain unresolved - people either find a way to make this Pokémon work outside of that strategy, or they will deviate to a different strategy that abuses the same unhealthy mechanic. For the former, it would have just made sense to ban the Pokémon. For the latter, restricting the Pokémon doesn’t actually accomplish much, and the council will be playing a game of whack-a-mole to try to add more complex bans. (“Let’s suspect Tera on Bax! Let’s suspect Tera on Valiant! Let’s suspect Tera on Moon! Oh wait, Gambit was broken even without Tera - let’s do another test on it to ban it for real!”) If this happens, the meta will be slower to approach stability, and there is no guarantee that the end result will be any better of a metagame than had we just either banned the Pokémon or changed the rules.
You can hear arguments that complex bans are not worth the overhead cost - accessibility, suspect timing, etc. - but I argue that, even accounting for these costs, the benefits just aren’t there. Do we really think that a metagame where Gambit, Volcarona, Espathra, and the like are legal but can’t Tera and everything else can is actually going to be a better metagame than if we just do something about the Pokémon or do something about Tera? Maybe it might be better than the status quo, but I don’t see the reasons why this route would make any sense when there are other clear paths to take.