Because dimension is about the connectivity structure of a set of objects, not the size of the set of objects. Sets with the same cardinality can have wildly d

Because dimension is about the connectivity structure of a set of objects, not the size of the set of objects. Sets with the same cardinality can have wildly different topologies, which describe the possible paths through the space, which in turn describes the nature of functions into and out of that space, as well as transformations and relationships within that space. A line through the center of a sphere doesn't necessarily touch the sphere in a "4 dimensional" connectivity structure, must touch the sphere in a "3 dimensional" connectivity structure, while a sphere doesn't even exist in a "1 dimensional" connectivity structure. The main reason that we use "4 dimensional" spacetimes for particle physics is that there are three independent components of motion with respect to time, and so it makes the math simpler to allow for independent variation of those parameters to functions, instead of compressing them into a single encoded number, eg, derivatives wouldn't function correctly (or likely even be defined) on a single parameter model. An additional reason that we don't use a "1 dimensional" spacetime is because the product of a "1 dimensional" time and a "1 dimensional" space is a "2 dimensional" spacetime -- and we don't have any models of reality that lack either space or time. (Which is good: any model needs to explain both, if only as an emergent feature.) Topology is about connectivity, not scale; dimension is a topological fact.