What do you mean by they are made on Earth?
It may be a little bit confusing but I tend to look at it this way... It is a little difficult for me to explain since I thought of it a long time ago, but correct me if I say anything wrong.
My point of view is that a person in this life learns the manners and obtains the qualities that are required in his/her future home. For example, all of our destinies are planned and Allah knows what our final destinations will be... However, we are here on Earth to fulfill those destinies. Jannah is a pure place where everyone has perfect manners, qualities, attributes, could it be possible that we learn those kinds of manners here on Earth. Not everyone with any kinds of qualities can go to Jannah. Only people who have acquired the qualities that meet the admission requirements of Jannah can enter it (just like schools, except your personal attributes rather than your grades matter here). So I was thinking similarly for Jahannam where there is all kinds of filth which admits people who have filthy qualities. May I put it like this "it does not accept good people" because it is not the place for them...
.....So I hope its not confusing for you but that is my analogy for life...Phew