Because changing the center of gravity is an expensive engineering solution. Crossovers came about as a loophole to CAFE (Corporate Average Fuel Economy) regula

Because changing the center of gravity is an expensive engineering solution. Crossovers came about as a loophole to CAFE (Corporate Average Fuel Economy) regulations, as CUVs are considered "light trucks" and are thus tested differently than pedestrian passenger vehicles. Since CUVs had to adhere to that "light truck" designation before the EPA just gave up and let them do whatever, often times that meant they had to have SUV like features such as taller bodies, higher ride heights, and thicker unibodies to enable towing. Lifting an existing sedan platform and creating a subframe for the rear was cheaper than engineering an entirely new platform with a lower center of gravity, and thus that's what they've been doing since 1999. Even as sedans drop like flies leaving only CUVs, SUVs, and trucks, they still engineer them as if they were just adding ride height to a sedan.