but nothing happens in an RPG unless you manually make it happen. You aren't actually expected to service your car by yourself, but you are expected to understand and apply the rules of an RPG whenever they come up.
If you look at third edition D&D, for example, the rules for grappling are widely considered to be more complex than we'd like them to be. The fact that you aren't grappling on most turns does not change the complexity of the grappling mechanics. That's still part of the game. ... but the total complexity of the game is still equal to the complexity of the grappling rules + the complexity of all other rules + emergent complexity.
Sure, 3.5e grappling is complicated. But, as you say, nothing happens in an RPG unless you make it happen. I have played entire 3.5e campaigns where the grappling rules never came up. The players in that campaign had no expectation to understand and apply the rules of the RPG that they never used. Having the potential to make something complicated does not mean the game has to be complicated. You can add modules on to any game to make it infinitely complicated, but the base system remains simpler.
Maybe I'll agree that the total complexity of a game is equal to the complexity of all rules. But the complexity of a game to one individual player is only equal to the complexity of the rules that individual actually uses. The 3.5 wizard doesn't care about grappling rules, because if they're attacked by a grappling monster the DM is the one who has to adjudicate. The 3.5 barbarian doesn't care about the complexity of the shape change rules, because it's up to the druid to keep track of their animal shapes. The rogue doesn't compare about the complexity of the turning rules, because the cleric is the one who will fight the undead.
Now, there is something be said about how modular a system is, which will help keep these complexities separate and allow new features to be added without affecting others. And there's a major point to made for the poor DM, who has to manage all the complexities of the individual players (and monsters). But to a vast majority of the players, there's simply to reason to call a game "complex" if they'll never experience the complexity.