The problem isn't in caves; it's mostly above ground at night. With people just starting out, I see them die several times within a short time period. Getting your first non-leather armor is really hard because you don't have armor to face monsters in caves and on your lot you die over and over trying to get your stuff back after the first time, probably losing many of your items along the way. Getting mobbed by 10 or so zombies, and a few creepers and skeletons can be impossible without diamond armor as well, which is the case on many lots. I have full diamond armor and enchanted weapons, and even then doing anything at night on my lot is a challenge. I have to spend the whole time trying to not die rather than getting any work done :L
(Sorry for lack of flow in the sentences, I was sleepy when I wrote this.)