It’s more to carry the load of multiple computers, instead of one computer connecting at 400 Tb/s think of it as a few thousand computers running gigabit connections over one single line.
You could supply an entire office building with internet with a single line from the provider.
What exactly is sending and receiving over such a link?
That has to be be a large amount of expensive fast RAM in the computers at either end trying to keep up with that.
Consumer-grade hardware is an order of magnitude slower, even for the good stuff.
Think more like backbone, a bunch of sources and requests all getting there faster.
It’s more to carry the load of multiple computers, instead of one computer connecting at 400 Tb/s think of it as a few thousand computers running gigabit connections over one single line.
You could supply an entire office building with internet with a single line from the provider.