Open source works well when the work is inherently cool and challenging enough to keep people engaged. Linux and Blender are two of the most successful open source projects, and the thing they have in common is that problems they solve are problems engineers enjoy working on.
Mapping intersections is extremely boring in comparison. The sheer quantity of boring work needed to bring open street maps up to the quality of google maps in insurmountable.
LLMs are freaking cool, and that bodes well for their viability as open source projects.