I don't get it why is Handlebars by The Flobots so sad?

It isn't sad. In the beginning its all the things you can do as a child, ride a bike with no handlebars, create a comic book, all innocent things. The second verse moves on to the good things you can choose to do with your life, create things for the good of humanity, and leads into the corrupt, the final verse is the corrupt, the bad in life, absolute power. Then it brings it all back to when you were yound and innocent riding your bike with no handlebars.