Why don't people boost back?

I sent out 4 today and only got 2 back, I see now why most people say they wont boost first, but I am curious as to why people don't boost back, especially after saying they will, it doesn't cost them anything and they can't use it for anything else, why not send back to someone who helped you, I just don't get it.

EDIT: THANK YOU TO THE TWO NICE PEOPLE WHO SENT ME A BOOST, I REALLY APPRICIATE IT