Why do you say 24GHz and not, e.g., 100MHz? (Apart perhaps from it being within a reserved band).

An antenna large enough to work at 100 mhz would be impractical. That's a wavelength of 3 meters, and incapable of reliable detection of objects less than about 0.7 meters in all dimensions (1/4) wavelength, not to mention the required size of the antenna array

Update: chatgpt told me that the wavelength (3m) is too large to detect humans, antenna size would be impractical, more power would be needed for transmission, and reflections are more of a problem than with higher frequencies.