Acquiring knowledge about the product takes time. Upstream has a better position just by being the one to create it and having all the knowledge about the product immediately, not after some time. Someone who decides to rebuild that would either have to fully maintain their own fork (and open source their work as well if the upstream has copyleft license), or upstream their changes, since reapplying bug fixes and new features requested by clients on top of the original codebase will take more and more time with each upstream change. Upstream can also restrict the use of their trademark, which would add a burden of marketing to downstreams as well.