Because that close the other person's eyeballs are giant, like bulbous bug orbs with big sweeping eyelashes that loom like batting rakes given the proximal perspective. It's like being attacked by a monster or something.
Or they think the other person is ugly and don't want to see.
Or, like tasting a fine wine or sumptuous dish, you just shut down other senses so you can focus on the tactile and taste experience.
The first time I ever made out with a girl, I had my eyes open for a bit of it (just didn't think about shutting them), well it's kinda awkward to have this face like.. 1/2 an inch from yours.. and at the same time you would feel stupid if she opens her eyes for a second and she catches you staring at her point blank haha.
No matter how gud, when u get that close everything tends to get out of focus n distorted. So everything that turned you on about ur kissing partner is visible in a different dimension that is never too flattering.So when you are already lip-locked it's best to let ur imagination run riot rather than count your babes freckles while she counts ur nasal hair while in the throes of a french kiss. Somethings are best left to imagination!!!
Oh you're all so crazy! Personally, it's because I want to make the most of that moment....almost like forgetfulness by closing my eyes to everything but the sensation of two lips moving together. It's a very sensual moment for two people who really love each other and closing your eyes just puts all the focus on 'feeling' rather than 'seeing'.