I have noticed on most gear list that people list and weigh the clothes they are wearing. I am curious as to why.

Couple of reasons.

What really matters is the total weight of everything. Also, proper clothing is critical for comfort and even safety.

To me a gear list, is just that. What is the total package that works for any given trip. Given that, weight becomes secondary.

Because this is Backpackinglight.com, where we're all weight weenies. And while you may not feel the weight of the clothes you're wearing as much as you feel the weight of the pack, you might still want to evaluate how much extra clothing you need to pack. You are "carrying" everything, even when you wear it.

First, the weight is being carried by your feet and legs, regardless of whether it's in the pack or worn on the body. That's why the total "skin-out" weight is the most important.

Second, weighing what you wear hopefully reduces "cheating" by carrying items in your pockets but not weighing them because they are not in your pack. These little items can add up! I personally include them in pack weight, but not everyone does.

Third, everything you take, no matter how small, should be on that list, whether worn or carried. That way you won't forget something important. If you have it on the list, you might as well weigh it.

Fourth, there are definite weight savings to be had in clothing and footwear on your body as well as in your pack, and you won't know what they are until you weigh the alternatives. Two examples in which I was able to make significant weight reductions: Switch from boots to trail runners, 1 lb. Switch from aluminum trekking poles to carbon fiber, 0.5 lb. Yes, I definitely noticed the 1.5 pounds afterwards!

Fifth, your total clothing, both worn and carried, should be considered as a single system, so you're not taking more (or less) clothing than you need.