It's really not that difficult to understand is it?
Firstly Motability is a charity that leases cars to people in receipt of the higher rate of PIP. That PIP, £77 a week, is paid directly to Motability to fund the car, the insurance and the maintenance. Many, but not all, of those on PIP would not be able to lease a car as they don't work, or have poor credit. The scheme gets PIP directly from the government, eliminating credit risk.
Secondly the disabled are exempt from paying VAT, so cars purchased by Motability are exempt from VAT, making the lease and maintenance cost lower.
So, to reduce the number of Motability cars you either tighten the qualifying criteria for higher rate PIP, or remove the VAT exemption. Removing the VAT exemption from the disabled would mean that crutches, scooters and wheelchairs would cost more. Adaptions to buildings would cost more. Maybe the Government could carve cars out of that if they haven't been adapted, but they chose not to do that. Probably because they, like you, don't understand how Motability works :)