You are the CSS framework.
960px is for people bad at math. Contrary to popular belief, a 960px layout is not the “ideal” width of a site. It is just a number that’s easy to work with that can be fully viewed in a 1024 x 768 monitor. There are plenty of “ideal” widths and with a bit of planning and a calculator it is easy to pull off other widths. What if your project requires some other width than the one specified by your framework? Not much of a time saver.
They are bloated, or perceived as bloated. Not every framework is a monstrosity like blueprint. Still, you will find “lightweight” frameworks like this and ask, “what the hell do I need this for?”. Then its trimming time, which wastes time.
Floats suck. See this post.
Irony? There are too many out there. Here’s a dozen of them and that was from exactly one year ago. Each purports to be better than the other. So what are the chances of you choosing one and sticking to it forever? What if you have different user needs like fluid layouts? You will spend time learning the pros and cons of a few of them, thus negating any time savings.
You can do a better job You know who can build a framework for your needs? You. You know what you need and don’t need. You are the framework.