Actually the number of developers ( and developer effective effort ) that would work on such a library is not constant. Once you get too many developers on a single project a larger effort is required to required to just for code / development syncing. The other issue is especially with open source developers is not all developers are interested in working in the same development models among other issues. Which means single projects tend to discourage some potential developers from joining in. There also is the issue of developers wanting to feel helpful a good developer will often feel more appreciated on working on a smaller project.
Finally just because implementations are separate does not mean that their is not cross pollination. Fundamental concerns found by one project are very likely to be searched for in other projects. Likewise with good ideas introduced in one project will often be picked up by other projects. Further because of a diverse system of libraries it is more likely that good ideas can be experimented with safely in different projects.