As I just mentioned, the interior is the place you will spend almost all of your time in. So all the character the car expresses through the outside suddenly will disappear because of how plain the interior becomes because you’ve seen it several times in other models. If you don’t like the interior design and layout of a particular model, the chance of it being in the other models too is now very high.
Having similar looking interiors rules change out of the equation to some extent. Having different interiors for different models is less streamlined, but gives more freedom of choice.