Because kg is the fundamental unit of mass and kW is typically used for electric motors.
Same reason you wouldn't use m²/s³ even though that's also technically correct.
Same reason you wouldn't use m²/s³ even though that's also technically correct.
Kilo is an SI prefix.
"The kilogram, symbol kg, is the SI unit of mass. It is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015×10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆νCs.[1]"
The base SI unit for power is the watt. The base SI unit for mass is the kilogram. Yes, this is dumb, but it's the way it is.
[1] https://www.bipm.org/utils/common/pdf/si-brochure/SI-Brochur...