Well, like others have eluded to before in this thread, a harem anime is all about the women, not the protagonist. Unlike what half of this tread (and the usual suspects at that) will have you believe, most people watch these shows not because they themselves want to be surrounded by beautiful women all the time, but because they get invested in the characters.
The protagonist becomes a vocal point for the love interest to focus around, he is often the reason they get or stay involved with the plot and already established characters. That's why most harem protagonists are more subdued. This focus on the female characters leads to most harem series having either a strong focus on character relationships within in the harem, or an overarching plot and characters goals aside from romance.
Good harem anime have cleary defined characters with strengths, flaws and goals; the women have more motivation than to get the dude and play off of each other, they have different relationships and chemistry with both the protagonist and the rest of the harem.
So in short, harem shows are the fighting games of anime: flashy vehicles for (mostly female) characters, where the story is not the main reason to watch it.
Granted, there are a LOT of bad harem shows, and they share the problem all bad anime share: formulaic to a fault. This is especially bad for harem shows, because they are all about character, and if you only have stock characters and cardboard cutouts, it is gonna be painfully noticable. But this is no reason to hold this against all harem shows or the people who watch them.
Are there sad, lonely people holed up in their room afraid of women and change? Yes. Are they a majority or the reason these shows are produced? No. I think most people know how emotionally draining it would be to have a constant stream of people around you who demand attention and affection. Saying that harem shows are mainly produced for otaku shut-ins is like saying Doom was mainly produced for people who want hell to invade irl, so they can kill as much as they want with moral justification. Heck, even the mostly ironic concept of waifus runs counter to this, because if it were people's fantasy to contantly be surrounded by the oposite sex, why would they pick favorites?
tldr: Are all harem shows marketed toward otaku shut-ins? No, BAD shows are. But there is crap in every medium, no reason to generalize.