A lot of people request that but is very difficult to enforce security like that because it becomes difficult to enforce SSL. With fixed pages we are able to force ssl if it is available because we have full control of everyting on the page, but if we allow users to add it to any page it becomes more difficult because if you force ssl on the page you often endup with some images or other content that is not secure because its just content entered by the user who created the page. The users using IE get a browser warning on the page about it having both secure and insecure content. I realize a lot of sites don't have SSL certificates and don't really have security but I don't want to build something that encourages bad security. I've noticed that very few big name sites do this anymore (have a login form on multiple pages).
I'm not saying its impossible to implement it securly but it is surely difficult. One of the best approaches to security is to keep things as simple as possible and having only one place where a user can sign in is an example of following that principle.