They were designed to be party games first and foremost though. I think it's different when you take a more serious game and try to layer wackiness on top. If nothing else, there's a huge difference in player expectations.

Anyway that's why I also used SC2 as an example. Blizzard's attempts to add more wackiness/variety to the 1v1 ladder maps have generally been poorly received.

Now, SC2's co-op mode has a bunch of wackiness and did great, but note that this is very VERY separate from standard ladder/melee games. It's not just "oh we changed a few rules" or "we took the base game and added wacky items" it's a much more fundamental rework than that, it's an entirely new 'vertical' for the game. And yeah, that kind of thing could probably work for a game like Rocket League.

It's probably more like 99% of players, but I think you're missing the point a little. What that commenter is getting at is that if your target audience are competitive players then it's not in your best interest to introduce "wackier" modes, as it isn't what those sorts of players want and might actually be off putting to them.

If your primary audience is casual, like in the case of Nintendo games, then it makes all the sense in the world.

Obviously you could argue that Psyonix ought to be appealing to a casual audience. That's a separate conversation, and I'd argue that RL is a fundamentally bad fit for that sort of game, but given what they have been going for thus far, not introducing crazier modes makes a lot of sense.