Like other people have said it's because it's so much harder to write but I absolutley love humor books. Have you read any of Judy Blume's Fuge books? They are the funniest things I have ever read.
'imitation is suicide....'-Ralph Waldo Emerson
To a philosopher all news as it is called is gossip and they who edit it and read it are old women over their tea.- Thoreau
Maybe it's because it's quite difficult to write purely comedy throughout an entire story. You tend to need to have a mix of genres. So there is comedic writing out there, it's just littered within a mix of genres. It's easier to right straight horror or straight drama...not necessarily better though.
'The creation of a single world comes from a huge number of fragments and chaos.' - Hayao Miyazaki
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