Phantasy Star Online was an extremely innovative game for the time, bringing the world of online RPGs to consoles like the GameCube. This was at a time when online connections were rudimentary at best, which lead to all sorts of unique design decisions.
In a Polygon interview with director Takao Miyoshi, we learn about one of the gameplay restrictions made based on slow internet speeds.
Yes, [program director Akio] Setsumasa handled all the server-related programming. He’s the one who said definitely no jumping in PSO. I concurred and that’s how we made the game. We had to decide what kind of navigation would be network-friendly. You might notice in PSO, it’s a lot like Diablo, where there are a lot of points that you can warp to. The reason why those points are so deliberately set is because if you’re playing online, there would be enough time between points where data could sync up and everyone playing would be in the same location. There wouldn’t be any lag where one player gets ahead of another player.