Because believe it or not, MMOs or even MOBAs for that fact do not require a huge amount of "bandwidth" and as per you asking for Mbps or even Gbps shows that you do not have the slightest clue on what you are talking about.
Why on Earth would you need to upload data to the server in Gpbs?
Go sit in the corner and think about what you just asked.
Think of it at the server end. That end needs a connection that handles the sum of all the traffic from the clients. Millions of clients means not much for each one.
Dividing users into instances is one part of that, but there's still a bottleneck at the routers that determine which instance server you're talking to.
I'm aware of aggregate bandwidth, thank you.
If the server can only handle Kpbs per client, it sounds like PvP (Cyrodiil scale) is under resourced.