[cma]

The real reason isn't bandwidth cost, it's visibility in the algorithm. You want a game trailer on your site: you can host it at a better bit rate for a great experience or use a competitor to. But if you do that, you get less views and thus less recommendations in the YouTube algorithm on your version posted there. So you out in a YouTube embed instead.

Same with steam or other dominate online stores with recommendation algorithms: you can market your game with ads linking a store with a lower cut. Less sales and views on steam from that means you don't take off in their algorithm. Game flops. Buy ads pointing to steam store and it would have kicked off the self stoking cycle and done well in this hypothetical. The other store can't compete with rate alone even in cases where you are driving the traffic, because you are giving up driving even more traffic at at the other store through the augmentation of the algorithm.