You learn a lot building something from scratch. And it's fun. You start with naive, clean abstractions and slowly encumber them with trade-offs as you make progress. That's a fun Saturday for a lot of us.

There's a massive difference between that and tackling some long-tail issue on a massive, mature project that apparently wasn't trivial enough or fun enough for anybody else to tackle. i.e. work.

Asserting they could've spent that time contributing to React is like suggesting they could've used that time volunteering at the homeless shelter peeling potatoes and doing jumping jacks. Energy and interest just don't work like that.