much of Western Europe

Correction added to my comment, it looks like that for most of Western Europe, fluoridation in water is not common. That was a mistake and I acknowledge that, and I have edited my comment to reflect that.

However, Ireland and England both implement the fluoridation of water. In particular, 73% of Ireland's population drinks fluoridised water

https://ukhsa.blog.gov.uk/2016/04/13/water-fluoridation-what-it-is-and-how-it-helps-dental-health/ (UK) https://pmc.ncbi.nlm.nih.gov/articles/PMC4081215/ (Ireland)

Additionally, so does Canada:

https://www.canada.ca/en/services/health/publications/healthy-living/fluoride-factsheet.html https://www.canada.ca/en/public-health/services/publications/healthy-living/community-water-fluoridation-across-canada.html

Why not? Or more specifically, why is this insufficient in the US?

Well, for one, tap water is readily available in the U.S., making fluoride super accessible to a large portion of the population! Also, fluoride toothpastes exist in the U.S. too, why can't we have both? Both are effective at preventing tooth decay, perhaps toothpaste moreso (and there are studies that show that, you can easily search for yourself I think), but both help to prevent cavities.