The answer is simple - Game Theory.
Like Hart points out, this is one business model of high-volume low profit. The consistent trouble with economics is that it assumes uniformity too often and that people are entirely rational creatures driven by cost and quality and quantity. In reality, if you say change the bunch of $100 photographers to $10,000 photographers and target a different market, it may well be the case where Photographer A offering a 10% discount gets less business because the clients at that end of the market may well be indifferent about the $1,000 discount and interpret the discount as a sign that photographer A isn't getting sufficient business for whatever reason and shy away from engaging his services.
Of course, I think the mix is pretty even. The people obsessed with costs (many of them out there) will naturally go for $100 photographers so your example probably gives a fairly accurate representation of the actual situation for that segment of the market. I sweat for the $100 photographers who are doing it full-time and don't have a way to raise their prices over the years though, because you would be stuck in a vicious cycle where you just keep working non-stop for low-value jobs. At 20+ years of age shooting events back to back on a daily basis is probably easy, how about 40 and beyond?
And from the other side of the fence as well, I'm sure we're aware that there are photographers who are charging way more than what their quality of work usually commands. That's because clients in the photography market aren't all about the images sometimes, but also "branding", "word of mouth", "how good this guy makes me feel", "he's more handsome" etc... Willing seller, willing buyer, that's life.
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