Batutta
Major Contributor
The limited fuel is a central plot point that dictates why they do what they do in the other galaxy's system and which planets they go to and how. (although, that may be the Endurance's fuel, not the landers) But, at the end, when they've ruled out 2 of the planets, they realize they cannot get home, but they use the remaining landing craft as boosters to slingshot around Gargantua to get Brandt to Edmund's planet, during which each craft detaches after running out of fuel.
They sent 10 or 12 astronauts during the Lazarus missions, and yet are only able to go check on 3. Fuel seems a major limitation for the smaller craft and so perhaps in getting it up to the Endurance from Earth, they use a traditional rocket to save the fuel for that gravity escape. Those small craft are capable of leaving earth without a rocket, but they only have so much fuel, and thus can only go up and down from a planet and thus escape gravity a couple of times.
Remember, on the first planet, Cooper doesn't even use any fuel in the landing until the very last moment. He uses aerodynamics to let the ship glide in and use the atmosphere to slow it until the last moment when he engages the thrusters for touchdown. He even says in response to a comment on his abrupt landing, "...yes, but very efficient." Fuel is a limited commodity.
Anyway, my thoughts on it. There may be other factors at play.
You make some good points. I will have to watch the film again to see if that information is conveyed. Part of it is the way the film is edited and paced. It gives the illusion they are flying around the solar system like the Millennium falcon when those voyages could have taken much longer. One of the reasons 2001 is still considered the most accurate representation of space travel is because of its pace. I'm not sure I would want Interstellar paced that slow, but speeding up the pace has its consequences.