But there are still real costs associated with running front ends like steemit.com, and the back-end infrastructure necessary to power them.
Because steemit.com was engineered like a traditional website, despite the fact that its database is decentralized.
Just look at html source of any steemit page, it will serve the steem content, meaning that your server is actually doing server-side rendering on every call to format the page and get the steem content in the html. In 2019 it shouldn't work like that. People should just download a static app and fetch directly the steem information from the steem nodes.
DTube website is served statically, like an image, and therefore doesn't have any the cost associated to serving the website (we actually use GitHub pages), even though we have other costs (video encoding & storage). Also we hit on your full nodes, but we could hit on community nodes or eventually get our own api booted up as Hivemind is sub 100GB ram requirement.