Because the personal itself is informal and brief; it's the first short step to the next short step the introduction. For example, "Gee, that's a nice boat you're standing on in your profile photo. Women are too smart to fall for them. If the person is into sports and you're not, then don't pretend you are. Make it easy for her to respond by giving her something specific to respond to. Oh, also, under no circumstances should your profile picture be of you playing a guitar.
Penis photos I've seen many many women explicitly ask not to receive these 2.
Step 8: How to respond to your first message
Both of which mean "close tab, move to the next profile. When you write to a woman, she'll read your message, see your face, and then with any luck click on your profile to learn more about you before writing back. I'm a gay man. I've sent entire emails that consist of "Drink? If your profile is full of monosyllables, make up for it with a more lengthy email.