First of all, using a cross product here is very far from the definition. If you familiarize with the concept of flux, this will be easier to understand. The current is often defined as the change of the amount of charge in a 'slice' of the conducting material at a given time, current density is a vector function that defines how much charge is flowing through at a given time, and the direction in which they are flowing. So current can really be defined as the flux of the current density through a surface, which is what that dot product implies.
This definition is especially useful for analyzing magnetic fields of a system. Since magnetic fields are created by moving charges and their direction depends on the direction of the moving charges, current density offers all the information you need to deduce the magnetic field that is created in the system.
Also note, a cross product here would output $I$ as another vector which is perpendicular to both the normal of the area and the current density. For a real world example, this would imply the current inside a wire would be perpendicular to the wire.