Speaking for myself, reasons why I don’t immediately share my own tooling:

Perhaps I want to hold it to my own standard for tooling, not the team’s.

Perhaps I want to write it in a language that the team doesn’t really embrace.

I might not think it’s worth anyone’s time to code review changes I make to my own tools.

I might want to do something dumb like bake an API key into the tool.

Maybe the project already has a similar tool, and I don’t want to explain why I am wrapping/ignoring it.

In short: sometimes the cost to collaborate on idiosyncratic tools is just higher than I’m willing to pay.