That's friction, and friction is deadly to business. Some people play with different groups of people regularly. Some people get started late compared to everyone else. Some people play in large mixed groups -- remember, MH is a game whose biggest market is teenagers, and it took off partially because people can play with everyone at their school, not just necessarily their 2-3 best buddies.
Each time you introduce platform choice here, you concretely lose money. Every person with two groups of friends that break for different platforms, you're looking at someone who might just not bother. When you can't get a monoculture going in large pools like a school, you're no longer dragging in new customers via peer pressure and trend-following because there's no longer a "buy this one thing and you'll be able to play with everyone" option. The factors that drive legs disappear and you split your upfront sales and then drop off the charts, or everyone just picks the preferred entry and standardizes on that and the investment in the second platform is completely wasted.
I mean, I don't know what to tell you here. Your entire theory about how this works is wrong. You want to use online FPSes in the US as an analogy here but the big differences between the two situations make it a really inaccurate analogy. Literally 100% of the appeal of this game is built around the idea that you buy it and play local multiplayer with people, so anything that makes that easier and more fun (rather than complicated and more work) will make the game sell better. ..
From a purely economic standpoint, the purpose of online play isn't to let you play with friends, it's to let you play with strangers. The goal is to make it possible for your customers to play with others even if their friends don't game, or own the game in question, or have time to play with you on the same schedule. It reduces friction by making MP always available: if you buy a game at launch, you're basically guaranteed to find people to play it with at any hour of the day, without doing any coordination or negotiation to get your friends on board. People can do the work to play with their friends if they want, but the feature has still served its purpose even for people who never once play with someone they know personally.
MH is tapping people into a much smaller pool of potential partners, which means they have to optimize for maximizing people's opportunities to play -- which in turn means making sure everyone can play the game with everyone else.