>Why make a green bikeshed when there's already a red bikeshed?
Because then you have two bikesheds. An important aspect of the fact that the bikeshed exists, as a separate entity not integrated with the house, is that you can choose a different one. Specifically, the one that's already painted the way you want.
Maybe you don't think this is a feature in your current circumstance. Others do, which is why it persists.
>why not try to turn it into a shared project?
This is basically both the highlight and the bane of the Linux world.
Why have another DE when there are already multiple ones? [0]
Why have another package manager when there are already multiple ones? [1]
Why have another distro when there are already multiple ones? [2]
So having another wiki makes perfect sense (or not depending on your POV)
0, https://en.wikipedia.org/wiki/Comparison_of_X_Window_System_...
1, https://en.wikipedia.org/wiki/List_of_software_package_manag...
2, https://distrowatch.com/dwres.php?resource=popularity