Because that’s not entirely correct. It does tend to go hand in hand, but there are other factors. Dry wines for example have more tannins that have an astringent property. They literally feel like they’re drying your mouth out. Same with like a dry vermouth. They have a very drying mouthfeel.

Sweet wines have the opposite effect. They do taste sweeter, but they also make your mouth water more and feel wetter in your mouth.