Swallowing your pride, merging into another project and taking a less glamorous role in that project is not as easy as it was to fork when steering your project.

I don't think it's because of the ego. But if you're working with other people, you need to do a lot of non-coding (non-fun) things. Align thinking, find compromises, establish and follow processes. Things are easier and more fun hacking alone. No processes to limit you, no one telling you "this doesn't align with the vision of the project" (and the other way round - you don't have to maintain code contributed by other people with use cases not interesting to you) etc. For volunteer FOSS contributors, doing fun stuff is often a big part of the motivation to give their free time to the community.