You're missing the fact that Starlink doesn't take your traffic to the destination. It takes to a nearby downlink station. Then it goes on those same optic cables to the destination.
It can take your data to the destination via inter-satellite laser links. That's how they offer service over water and near/at the poles, where downlink stations are not in line of sight for the satellites.
"can take your data" is not the same as thing as "takes the shortest path". There is no way that Starlink is using precious inter-satellite capacity when your data can go direct to a downlink station and transit on dirt cheap fiber