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” etc.