Why was the Ranger launched from Earth on top of a rocket?

In the movie Interstellar, the crew of the Ranger launches from Earth from on top of a rocket similar to the SSL. They are on their way to rendezvous with the Endurance spacecraft that will take them on their voyage, so the initial assumption was that the Ranger craft was not single stage to orbit capable.

However, later in the movie we discover that the Ranger space ships are indeed SSO capable as they are able to lift off from the potential colony planets with no external assistance. Further, the Ranger is able to lift off from Miller's planet, which is 130% Earth normal gravity, without any assistance.

It's clear that the Ranger could have lifted off from Earth without the rocket assistance, so why didn't it?