Women are generally a little more self conscious about how they look due to a lot of models and TV stars setting a so called good example of the perfect body. And when you put who you are into numbers.. What your body is like... It becomes more real and sometimes slightly more embarrassing.
Because it's rude. Unless you have a good reason, like you're their doctor and they have an appointment with you, it is none of your business what another person weighs. If you ask a chick how much she ways, she'll probably assume you're going to judge you about it, whether she's normal weight, overweight, or underweight. And saying that she should lose weight is even worse. It's insulting and demoralizing.
It's rude and offensive. Quite frankly, it's non of your business how much they weigh and if they can lose weight. If they're happy with themselves, that's all that matters. You don't know their circumstances. Maybe they have problems with their weight and have problems fixing it. If someone (random person) were to ask me how much I weighed and if I would lose weight, I'd flip. I know I'm not thin, and I'm working on it. I don't need a random person to remind me about it. It's none of their concern. :o