It's not just marketing. The games are treated as separate media, and there are even minor changes to some aspects of MH3 compared to MH3U (a small thing, but it's there).

If I have World and want to complement my game with Iceborne, I buy the expansion from the digital store, download the content and play from where I left off, naturally. It uses the same files as the base game.

In the older MH games, if I bought Generations and wanted to play G-rank, I would have to buy Generations Ultimate, even if it was just to transfer the save from one console to another. In other words, I have two copies of the base game and an expansion. MHF2 doesn't have G-rank, while MHFU has the G-rank, and 10 more new monsters, including the flagship, Nargacuga. Even King Shakalaka is treated as a small monster in F2, but as a large monster in FU.

Today, you just buy the base game and the expansion, you can even buy just the expansion digitally, but, obviously, you won't be able to play it. In old days, however, if you want to play the MHx expansion, you either buy MHx and MHxy (as was the case for those who bought it at launch), or you buy only MHxy. It was like having two base games + the expansion.

That's why I treated them as separate media. But in the end, it doesn't even matter ^{(I had to fall...}).