I suspect we'd have more luck forming teams, if the people forming the teams aren't devs. Devs always think, "I'll just build it all, myself." Someone who needs help building, will look for help.
Discussion
Yeah a lot of devs don't naturally have product owner/project manager skills. Collaboration requires the product owner to precisely define the work to be done, to delegate it, and to validate the results. This can be done with project teams or it can be crowd-sourced via bounties/contract work.
Part of the problem is related to funding. Projects that have a lot of funding should set some of it aside towards bringing on extra devs for collaboration (and some are, Damus and Amethyst both allocated some of their grant funds towards expanding their teams or paying out bounties). However, when a project is only funded enough for one dev working on it full time, there's not going to be any cash left over to support additional contributors.