For the same reason they reject so much bad work. They are innundated with many, many more submissions than they can handle.
When I started out I was one of four "first readers" who read manuscripts from new authors at a literary agency. We all had a couple dozen manuscripts on our desks every morning, and were told to read no more than ten pages of each before either rejecting the work or passing it up the chain to the boss. On a typical day only one or two manuscripts made it up the chain from among all four of us. Most of those were rejected by the boss before he finished reading fifty pages.
Other than the brief time it took to read those fifty pages, the boss was busy with the nuts and bolts work or representing the authors in the agency's stable and launching the very, very few new authors who were selected. Submitting to publishers, negotiating paperback rights, trying to sell film or TV rights, selling foreign rights, trying to attract reviewers' interest, attending any signing or launch parties that were in New York or close enough to reach conveniently, checking royalty reveues, meeting with the authors and all the rest -- plus managing the office -- was a full time job.
One new author every month or so was all the boss had time for.
The math worked like this: Together, the four first readers screened about 100 submissions every day. That's about 20,000 a year. The agent reviewed 20 or so each month, so just 250 of the 20,000 each year. That's 1-1/4%. Of the 250 reviewed by the agent, only 10 might be accepted. That's just 5 one-hundredths of one percent of the original 20,000. Of those, only half would eventually sell to a publisher. That's five out of 20,000, or 2-1/2 one-hundredths of 1%.
The decisions, from the first readers to the agent to the publishers, were made on one criterion: saleability. Badly-written genre novels were accepted if they adhered to the conventions of the genre, because pub;ishers -- and readers of that particular genre -- would buy them no matter how atrocious the writing. Beautifully-written books without commercial appeal didn't make it past the first readers' ten-page limit.
Were there literary masterpieces burried in the reject pile? Almost certainly. Were commercial blockbusters rejected? Probably. But the system was actually gave writers a better shot than they have at many agencies today. Back then writers got ten pages to make their case with the first readers. Today, many agencies won't even look at an unsolicited manuscript. So the 5 out of 20,000 chance a new writer had of getting published a few decades ago has shrunk to virtually zero today unless she or he self-publishes.