Because if it bores him, he can assume it will bore his readers. Substitute ''stupid'' or ''pointless'' or anything else for ''boring,'' if you don't think that's the right word, and the result is the same. For me, stupidity and pointlessness and all the other possible flaws are subsets of being boring, but maybe that's not how other people use the word. When someone tells you that if you start your story with a cliche the editor will reject it, he's generally telling you the truth. Cliches are not bad because they're cliches.
Cliches are bad because they make editors reject stories.
Keep cause and effect the right way around here. Cliches are bad because they bore editors. If they don't bore editors, then they're not bad, and there's no reason to avoid them. I should note that this brings us to the single largest advantage established writers actually have over beginners. These are good writers, but I really don't think they make any difference to sales.