Because the measured parameter is scattered light intensity. Volume would need to be derived - it is not measured . In the case of zeta potential then it’s movement (mobility) that’s measured and ZP will be derived based on assumptions (which may or may not be valid).
In addition to Alan Rawle's correct answer, you might also like this blog on intensity volume transformations for particle size, with some references: https://www.materials-talks.com/intensity-volume-number-which-size-is-correct/