I'm really don't know much about the technical details of bitcoin, but why is a bitcoin address tied to a specific key pair? Why isn't the address+balance just

I'm really don't know much about the technical details of bitcoin, but why is a bitcoin address tied to a specific key pair? Why isn't the address+balance just signed with a key pair? That way me knowing a key pair wouldn't get me an address with a balance in it... is there something i'm missing? EDIT: I guess it doesn't matter, since the address space is so large. Either way, if i were targeting an account, i would know what key pair to attack..