Certainly everyone who creates a (somewhat serious) web framework is at least aware that they could also participate in the development of an existing on. If they feel there is something to gain by going from scratch, why not? I don't think the popular existing frameworks exactly suffer from under-contribution.
My point was that it's fine if someone starts working on a new framework, but personally I would rather have all that effort be used for developing services or tools that are actually needed and do bring value to the community.
EDIT: To make it more clear, the issue is when the creators of such frameworks do believe they are going to change the world by recreating in a slightly different way something that already exists. If they are doing this for fun or to get experience, it's all good, but very often those frameworks do seem to have the ambition of becoming the next big thing and revolutionizing the way people are developing web apps.