Crimson Longinus
Legend
You assume there's anyone actually doing that. As @Chaosmancer said, many players simply think 1st level is just the place you always start. And many DMs have no interest in changing their minds about 1st level being the place you start, so it creates a self-perpetuating cycle.
People do lots of things not because it makes them happy, but because it's just "what one does." That doesn't mean they hate it either! It just means that you cannot say, "Ah, yes, people do X, therefore people like X."
So if we assume that majority of people are so blind to their own interests, if we made the default starting characters to be (effectively) higher level and the lower level ones optional, how would this change matters? We would have just created a new default these players would now unthinkingly flock to, regardless of whether they actually liked it or not. It just so happens that this new default is the one you prefer!