Because bandwidth isn't the only AWS product, but you can't buy bandwidth for cheap from XYZ Inc. and connect that to your AWS account. That's not technically possible, you have to buy it from AWS.
So instead of using the cheapest vendor, companies use AWS for a ton of reasons (broad support, everyone knows how to work with it so it makes recruiting easier, hundreds of features, etc...) and their high bandwidth bill is worth it.
So instead of using the cheapest vendor, companies use AWS for a ton of reasons (broad support, everyone knows how to work with it so it makes recruiting easier, hundreds of features, etc...) and their high bandwidth bill is worth it.