Mass is only part of the equation. If you move a golf cart at 45mph it's still going to do damage if it hits someone. We have to reduce speed as well and that means it takes longer to get to your destination. There are solutions there (mixed use/high density residential?) but it's not as simple as "smaller cars".

The 1000kg vehicle protects the 80kg person from harm because the 80kg person is not designed to go 100kph.