Bguy7
The Dragon Lord
That's... exactly what being optional does? If we define "the main game" as what needs to be done in order to get yourself from Postwick up to Wyndon and back, again, in what way are Max Raid battles "main game content"?
For years, we've been able to mix records or spin Poké Blocks or get difficulty keys from someone else's game or collect Friend Safaris or lord remembers what other elements we've gotten that encourage you to interact with other players. The rewards for Max Raid battles are far more lucrative, sure, but how are Max Raid battles any more "main game content" than any of those other elements, aside from the fact that they utilize this generation's new mechanic?
I would say that the major reason is what you said yourself at the end of your post. Gigantamax Pokémon is large part of the game, and Max Raid Battles are the only way to get them. Yes, it's optional in the sense you can ignore them if you don't care about Gigantamax Pokémon, but the same could be said for any new Pokémon or form. The things you listed all tend to be features that are used to enhance your experience if you so chose to go out of your way, but you aren't missing anything if you don't. Here you are most certainly missing something.
I'd also argue that the way the mechanics are implemented and integrated matters as well. The things you listed tend to either be things done in obscure corners of the game, are post-game exclusive, or accessed only through menus. That removes them from the core experience of the game. Meanwhile, Max Raid Battles are presented front and center in the Wild Area, and touted as main focus for the place. In more precise words, they are integrated in a more prominent way.