For efficiently-hosted sites with little media it's not too bad. E.g. hosting a static site just doesn't cost much, even if you're hammered occasionally.

That's extremely far from all sites though. It's probably safe to say it's a severe minority, particularly when you ignore personal / non-profit-bringing sites. Tons of small and large sites run stuff like poorly-written wordpress or ruby on rails or thousands of microservices doing god knows what. A major increase in request volume on those can easily mean significant increases in hosting charges (e.g. small-% on big, many multiples on small) or significant effort in optimizing (which is expensive too).