All rest of comments are great/good and ok. Reasons of Environment? Love? Life? Empathy? and Taste? have been addressed in well formatted words. I have a slight different overview, i.e. Efficiency.
Just like we expect our machines to perform the best in minimal resources (Tesla). We ourselves can perform in minimal resources. Making an imaginary limit to our wants and playing within our needs. By doing so we can easily address all the above reasons mentioned by others. Also now we know that the shared resources are limited in one way or another.
Balance diet as required by a human body is available in nature. It is completely upon an individual whether he wants to be Tesla S or Hummer.