In an ideal world yes these sites shouldn't ask for more than the basic information when you sign up.
However, a lot of sites are built for marketing purposes and one of their goals is to get personal details that they can use in their marketing or pass on to others.
So you have two choices:
Don't sign up.
Sign up, give your information and make sure you set the "don't spam me" and "don't sell my information" options the right way.
If you are really bothered you can give false information as most of the sites will just be checking for some input.