Because they are selecting proposals for the program, which includes a stipend (https://developers.google.com/open-source/gsoc/help/student-stipends) of $1,500-$6,000 for the summer. So the process looks a little more like the process for applying for an internship.
I do this on Hyprland all the time, but it’s a tiling window manager. I’m not sure any desktop environments have support for it.