Largely because of the need to make a living.
For example I love art but i can't make money from it and was not encouraged to pursue it due to the need to make a living.
I am about to embark on a career that will get me good money but I am not as passionate about it as art. Also i would love to be at home working as i hate politics.
When I start working i wont have much time for hobbies. Ce la vie!