There are plenty of reasons to reinvent tools that have existed for literally decades and are included in pretty much every Linux distro.
1. Someone wants to do it. Most open source is developed because someone just wants to.
2. You aren't happy with the language it's written in C vs Rust.
3. The tool is lacking some set of features that you want.
4. The tool isn't as easy to use as you would like.
5. The tool doesn't support some platform you care about.
6. Using the tool requires more knowledge or cognitive load than you would like.
7. The tool you are replacing has multiple variants that have different feature sets slightly different supported syntax (gnu make vs bsd make).