I agree with you, but getting the current maintainers -- who are technically, not maintaining the libraries -- to yield maintenance at least to the whole community (or to a larger set of contributors) hasn't been "proven" yet at least for Boost. I know Boost.Thread had been taken over successfully before, that precedent required having to track down the (sole) original maintainer, getting him to agree and yield to a specific person, sounds like a lot of work compared to just people clicking a button to "fork". :D
Boost.Thread was only complicated because of license issues, for a library that's under the BSL, the original maintainer can hardly complain if someone takes it over (with the consent of the mailing list) while they're AWOL. Just my 2c... John.