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.