Why is 一口 ひとくち and not いちくち?
くち is a kunyomi, like ひと, while いち is an onyomi, so I feel like the other way around is what would require explanation.
You of course can have mixed on and kun readings, but they might “feel” more natural when they match.
As Leebo said you can get exceptions to the rule in many cases, but one tip I heard which specifically addresses your reading (and one I generally find holds true) is that when you’re pairing a kanji with a body part they often both end up as the kunyomi. Like for exits, 東口、北口 etc are all using kunyomi.
Some other examples I can think of are for right and left arm, 左腕、右腕、and for humming 鼻歌, they all use kunyomi. I’m sure someone can come along and find some exceptions to this however lol.
Exception, also read as on’yomi compounds. さわん and うわん. But the bodypart rule of thumb is a good one. I would however advice you to learn words as they come, and not rely too heavily on finding rules.