This might be because they’re not interested in maintaining it for future iterations.
I totally get it that a maintainer doesn't want to merge some code, even if it's totally working. Is the new feature a niche case, or has if a wide audience? Is it aligned with the project goal/philosophy? Will it be easy to maintain/evolve/debug? Does it add unnecessary/unwanted dependencies? At the end of the day, the maintainer will be the person that have to work/support with this code.
Please keep in mind that if you've spent an hour on a project, maintainers have have certainly spent hundreds if not thousands more time on it.
The reason I open a PR is so that it becomes public.
Most of my PRs solve stuff that I wanted to solve. For example at some point I added simple math expression evaluation in the Start menu of Linux Mint. It worked, I use it in my computer and I published it as a PR. I Think it was rejected because of so e bureaucracy. It's OK. It's there if someone finds it useful, I do t care doing bureaucracy, but I understand why they would want that.
Normally the larger the project the more "bureaucratically painful" it is to have a PR accepted. In small obe person projects, .y experience has been that original devs are surprised and happy to know that someone used their code s d is bui5on top of it.
and the beauty of opensource is that the fork button is right there! You don't even need to ask for permission.