More specific reasons were. Legal boundaries between ownership and taxpayer money separation. Of cause GDPR is also a big obstacle in joining forces on this scale.

I worked on a project which tried to make a joined venture between several municipalities. I'm not sure if that partnership has ended yet, but overall you could only progress if there were people on both sides collaborating with a "better ask forgiveness, than permission" mindset. As soon as the municipality lawyers or their security officers got involved, all progress stopped immediately.