The question can be turned around. Why can't the hardcore be satisfied with the exclusive content catered to their playstyle and allow the casual player base to enjoy the content catered to their playstyle? In most of these arguments, it's not the more casual player base that never touches that content that's asking for more. It's those who do the optional hardcore content that get upset when they get bored in casual content and want to gatekeep it away from casual players.

If hardcore players want to make that argument that that optional content needs to be made harder, that's fine. That is a valid point of view. But often the argument is that the MSQ content is too easy and needs to be harder and the hardcore don't understand that that content is made with a wide variety of playstyles in mind and is not meant to challenge them. They will need to seek out different options to suit their playstyle the way the casual player base seeks out different options besides continuing on to Extremes and Savage and Ultimate to keep themselves busy in the game.