Because you get dressed, look at your short self in the mirror, and say, "Everything's covered, good to go." Then you go out and you have men a foot taller than you with their higher vantage point, and suddenly
every top is low cut. Women's clothes are made that way and it's very difficult to find clothing that's not low cut and/or see through (tops AND bottoms). I've gone to class in men's jeans & a pocket T and caught men leering down my shirt. It's constant and it's very tiresome...
Speaking as someone who's 6'8, and thus at least a foot taller than most women, I can have some sympathy for that view point, certainly a men's v-neck can reveal more to me than may have been intended.
That being said, you can't tell me that a plunging neckline is designed to do anything but divert blood from my brain, or that a draping neckline isn't designed to distract me. Hell, even a sweetheart or surplice catches my eye more often than I'd like.