Anyone could easily make high level adventures, but none of these people or companies are charities. It doesn't matter how big or small they are. Just because a few people might want something doesn't mean anyone else needs to spend their money to give it to them out of the kindness of the hearts.Unlike 3rd party folks who are very dependent on their sources of income and generally don't have the resources for an anthology book, WotC could easily make a high level adventure or two in each of the anthology books they put out. Folks needing low and mid stuff would still eat it up, and the rest of us could get some high level adventures.
And besides... as we have seen with the responses to all the high-level stuff that WotC has produced... no one seems to like any of it. So I don't know why people would want WotC to make more of that stuff for them anyway because they'd probably hate all of that too.