Everywhere I go, I am asked to create a new account. Why don't all Stack Exchange sites have a single login credential and reputation carrying?
They do. It's called OpenID. It's username / password from a single provider - Google/ Yahoo /Stack Exchange. You can use the same across all Stack Exchange sites.
I don't agree, and this has been shot down before. Repuation is a measure of how much the community trusts you - just because you have semi-decent programming skills doesn't mean you will have the equivalent server admin skills and as such you much prove that you can be trusted. A separate rep system per site ensures that.