Because ceramic is an extremely vague term - as a parallel you can look at carbon which can be crumbly and a good fuel, one of the hardest materials on earth or a variety of other forms.

Well, the bottoms of cleets are friction-inducing surfaces, but unless you pour a LOT of soda onto them, they ain’t gonna be sticky.

It’s as simple as that.

Quoting Wikipedia in reference to the variance in ceramics for the degree of crystallinity and number of electrons in their iconic and covalent bonds:

With such a large range of possible options for the composition/structure of a ceramic (nearly all of the elements, nearly all types of bonding, and all levels of crystallinity), the breadth of the subject is vast, and identifiable attributes (hardness, toughness, electrical conductivity) are difficult to specify for the group as a whole

Ceramics are a broad class of materials with a wide spectrum of properties. There is no one singular material called “ceramic.” Ceramic materials can be made smooth and slippery or textured and grippy, and everywhere in between.

To put it simple: fine china is ceramic, but so are bricks.

Ceramic comes from the word Keramic which means relating to Pottery.

If you mix sediments with moisture and fire it in an oven, it is ceramic. Difference of materials means different properties. Some materials crystalize, some burn, or some polish smooth easily.