Because big rockets are useful. The rocket I used for my Team Titan entry should be enough to complete all challenges, after a few modifications. I'd rather not have to re-design my entire rocket every time I do a challenge, so it's nice to have a large, general-purpose rocket.
Try to accomplish Moon landing and bringing the astronaut back home alive at Realistic level of difficulty with that "right rocket" in your picture.
Then you will know why.