For the same reasons you make
quick judgements about what you think you'll like — or not like — agents make
quick judgements about what they (and other readers) will like.
I'm not sure how this illusion got started, but I'm convinced it materialized along with the popularity of online dating and the superficial and «instant» way of
judgement that helps us make a
quick determination of whether or not someone is datable by reading a couple of paragraphs
about them and glancing at a few (often bad) photographs.