so why don't we do it?

I think the answer is pretty simple. Time and Money. We don't have enough time. Growing up we realize that we can't do everything that we want. A lot of people pour a lot of time and effort in their work even if it isn't what they want to do. Parents sacrifice their time to take care of their children.

I want to do a lot of things like play the guitar/piano, learn Japanese, get better at sports, etc. But not only do I not have a lot of time to learn and do all of them, but they also won't help me get more money for basic needs.