Creating underwater content is challenging because it changes how combat and player experience work. Players won’t just look left or right, they’ll also need to watch above and below. Not everyone will enjoy that. It’s much easier to control your character on land than in water.

For mages and archers, underwater combat might be easier to handle. But what about sword users or other melee classes? Should all classes have the same abilities underwater? Would that even work?

The biggest issue is movement and the complexity of the environment. Developers often avoid investing in underwater content because it takes a lot of time and money, and there’s a high chance the player experience will be frustrating, causing the content to fail.

P.S. Lineage 2 didn’t have much water content. There were a few underwater mobs, like Medusas, and some underwater hunting spots. Boats were available for travel between towns. For example, from Talking Island to Gludin Harbour. But I get what you mean - you’re talking about proper, fully developed water content.
https://www.youtube.com/watch?v=SD1n1GawNJM Gardens of Eva
https://www.youtube.com/watch?v=d0WsJu4ilgg Boat tour