Because while all those "conveniences" are available you are not at the mercy of more opinionated frameworks like Rails. You are free to use them or add your own or use none.

I don't have time to do simple request/response building although if I did Python's Paste/Webob would be my choice so I choose frameworks that I can add in the usual "extra" requirements that I do need.

This blog post is a couple of years old but sheds some light on the abundance of frameworks in the Python world at least. The closing section sums up the difference between full stack frameworks and lighter frameworks.

http://bitworking.org/news/Why_so_many_Python_web_frameworks

Another term I've seen around is "utility library" instead of framework. Perhaps that is better?