> Why? If it is well-designed, useful, and has no obvious technical flaws, why shouldn't it be included in open source software.

I think its quite easy to find examples by thinking of the extremes.

- Why don't git add a native UI? (out of scope)

- Why don't excel add lua scripting? (already has visual basic)

- Why don't neofetch add a built-in ascii art editor so people can more easily customize their logo display? (Bloat)

- Why don't pandas and numpy just merge? (confusing user experience)

They can be amazingly written, with impeccable docs and test suite. But they're out of scope, deviate from the project philosophy, confuse the user, add maintenance for the future, or could could be their own projects.

> - Why don't git add a native UI? (out of scope)

Git has native UI, just a bad one just like its cli UI, so it is in scope. You've just out-of-scoped better user experience.

> - Why don't excel add lua scripting? (already has visual basic)

Visual Basic is a bad/obscure language. Even real Excel didn't stop and added some JS/Python support. So you've again just rejected better user experience, very nice "project philosophy"!

Sure. Although git has a simple native UI, and there are many frontends, like Git Cola, lazygit, and so forth, although only the first is developed by git. The others are welcome. It is definitely out of scope for git itself.