LogFAQs > #969299107

They're technically not required to go to different planes and can still do three set stories. Thay's basically what they're doing with Dominaria United, Brothers' War, and Phyrexia.

But blitzing through different gimmick planes is probably better from a marketing standpoint. It's easier to sell things if the multiverse is treated like a theme park with the Fairytale Plane, the Viking Plane, the Magic School Plane, and the Cyberpunk Plane.

It's the same reason Planeswalkers are pushed in everything (and why they're almost all human). It's easy to market them and to create merch/etc for your fire mage superhero who will make an appearance on every other plane you go to for the next 15 years. It's not as easy to get players attached to characters whose stories entirely take place on their plane and who can't simply hop to the Cowboy Plane or whatever comes next.