Not a physicist -> My assumption is that this is because x,y,z are just constructs to model physical space. They don't exist and we could just as well describe

Not a physicist -> My assumption is that this is because x,y,z are just constructs to model physical space. They don't exist and we could just as well describe space with cartisian coordinates or some other system where the dimensions would not be isometric. Meanwhile, as you point out, t as a vector is also itself a convenient model for something, not an actual dimension.