The high bandwidth price is mostly to drive away seedboxes and porn sites. Huge discounts are likely available to big customers.
It's most likely vendor lock-in and people are dumb enough to pay for it.
There's no other rational explanation why Azure or Google aren't hammering AWS on bandwidth cost as a competitive angle. They're basically conspiring in the industrial sense, no different than airlines that somehow magically agree to simultaneously raise prices or department stores that for decades all agreed not to discount the perfume etc. box section of their stores.
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But it's not fair to say that price from Google is high just because some random hosting company gives you unmetered 500mbs for $99/month. As we have seen with many unlimited cloud storage offerings, some companies do unsustainable offers to attract customers. There are different strategies behind this. Some just count on that majority of customers won't use everything they could. Others know the price hike is inevitable, but believe customers will still stuck.