>people have already built most of those microwave tower paths here on earth

I can assure you that lucrative paths like New York-London and New York-Tokyo have not been built yet. In fact, the vast majority of lucrative trading paths have not been built yet due to geographical restrictions. Yes, fiber optic links exist, but those only go up to 0.7c.

To a trading firm, the choice between a satellite link that does ~0.9c and a fiber optic link that does <0.7c is obvious.