Why not just use mkosi? There are so many tools that do this already and mkosi is cross distro and can do super modern stuff like use UKIs.
sometimes the answer is just, why not? - it's not forbidden to learn something or try a different approach.
2. Contributing to a project that almost does what you want leads to better tools that are also maintained so they can be relied upon to build new things.
3. Instead of building the Nth version of something, you could be working on something novel.
Theres no value from building something new if you can just learn how something that exists works and contribute to it instead. NIH syndrome is actively harmful to open source.