Question

why isnt there a standard SI unit for volume?

Answer 1

There IS, the cubic meter, m^3. The basic SI units in the "MKS" system are the meter, the kilogram, and the second. Derived units such as those for volume, velocity, force, energy, etc, are "SI" as long as they are based on the basic SI units.

Example: The energy unit "joule" is equivalent to,
newton*meters. One Nm = 1kg•m^2/s^2, a derived unit based on the MKS system. The calorie is also an energy unit but it is not based on MKS.