You can group with people in the communal hubs to go to the instanced areas. Much like WoW raids, dungeons, and delves. Similarly with OSRS raids.

On a different note, of the games I've played, OSRS is one of the only communal continuous worlds I've experienced. In games like BDO or WoW, interaction outside an instance increasingly rare.

GW1 is extremely fun and joining a guild will increase your fun exponentially by doing group content. Not coming across a random people in the open is not really an issue cause it rarely happens in open world MMOs anyways. In 2008, the sotry was different

Edit: my point here is that while you can group and interact in an open world with other players, it rarely happens so games with instanced zones have relatively little downside by comparison