I don't really want to be that guy, but at least in the case of React and NextJS, the companies have a business reason for them being open source.

Meta uses React internally in a lot of projects. Every other company or developer making contributions in their spare time is free labor, and it directly improves Meta's own products.

Vercel has a vested interest in having developers adopt NextJS, as they sell web hosting. It's easier to build a community of developers around an open-source product, and they even help out with contributions, documentation writing, and QA.

My point is more that all these open source tools have been used by many, many, many sites to build a series of black boxes on top, for which there is no source available. I suppose one easy example is the existence of EME in open source browsers, the existence of which being the reason I actually don't run a pre-compiled binary of Firefox, instead building it myself, with EME not built in.