I've never understood it. I've only had horrific experiences with my open source efforts. From the simplest cases like being ignored on PRs and tickets to mockery in social circles for algorithmic mistakes to finding out my work was enabling bad people to finding out people were misrepresenting my employment status while using my work in violation of the license, I don't think I've ever once had a positive experience…
You're not entitled to having your PRs merged, or even having anyone look at them. PRs not of particular interest to those maintaining the project simply get low priority. Especially with volunteer work, time is limited and while your PR might appear large and of importance to you, it may not look like that from other perspectives.