The solidness is an indicator of quality. Apocryphally Mercedes used to add lead weights to their doors to get that solid 'thunk'. Nowadays car manufacturers are a little more subtle at it -- there's a lot of engineering that goes into how a car door feels and sounds to give that impression which is very much intentional.
AFAIK a lot of engineering goes into the hinge to make a car door feel heavy. Meanwhile the average person can easily carry two around without too much effort.
