Regardless what you may think, I DO know when and where to place commas in a properly formatted sentence. Problem is, when I put on my author hat, I don't always properly format my sentences. Sometimes my sentences are not sentences at all.
Few people use properly formatted sentences when they carry on casual conversations. Reading a novel should be like being on the receiving end of a casual conversation. Sort of. Creative use of words is an art. Alternating long and short sentences helps keep a paragraph flowing, assuming the paragraph is delivering something useful or entertaining. Entertainment is useful, right?
Don't you hate it when you watch a commercial where all the "real people" talk like they are reading a paper for English class? Do you sit there and shake your head and say, "real people don't talk to each other like that"? I do.
Sometimes my fiction omits commas because I don't want the reader to pause just yet. Sometimes I use a period instead of a comma. Commas say slow down. Periods say stop. Exclamations say stop dammit! Art.
Until a few weeks ago, I thought the practice of omitting commas in fiction was a fairly new trend, but I picked up an old copy of The Grapes of Wrath for a third (fourth?) read. John Steinbeck published The Grapes of Wrath in 1939, and he didn't use a lot of commas. Perhaps there was a shortage of commas in 1939. Maybe, just maybe, William Faulkner used them all up.