Because your logic is flawed coming to that question. It insinuates that one is needed over the other. This is not a fair comparison because toothpaste and fluoride suppliments are purchased and flouridated tap water costs almost nothing to consumers of municipal water. All sources must be taken into account for public health programs like this.
We're not comparing with fluoridated tap water because that's not something you can control as an individual. At most, you can put pressure on your city to add it in. Until that happens, and even after that happens, you still need to put in work to care for your own teeth. It's very unhelpful to tell someone living in a city with unfluoridated tap water to just choose fluoridated tap water. They do not have that choice.
I grew up with well water and we didn’t have naturally occurring water.
Our doctor prescribed us a chewable tablet we took every night after dinner.
Didn’t get my first, and only, cavity until I was 35.
My ex-wife who grew up with me had a dad who was “they’re putting gay vaccines in our fluoride to turn our cavities to queers” type and they didn’t …. Cavity city.