I think people just don't like it because it's too hard for them, but games usually have some sort of challenge in them.

They make 2 new GATEs each expansion usually. It's part of the regular content cycle.

It's popular content among sprouts because it's very low level content and one of the few interesting things they can do at the start of the game.

I think the developers just enjoy the Gold Saucer and the idea of it, because Mahjong was just a passion addition to add lore to the game (that Domans play Mahjong) and not exactly something everyone wants to queue for like GATEs.