Sadly, I do the same at times.
I just want to hack together something that works for my use case. If I can share that with the wider team, that is superb. But I really don't enjoy my hacky utils being subject to tedious code review scrutiny they don't deserve.
To be clear, I _do_ add utils that I think will be useful to the common repo, subject to review, and mostly they will be merged. But if it gets painful, I just retract the PR and keep it locally.
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.