Mad Mike wasn’t a flat earther. He was just scamming flat earthers to fund his thrill seeking. His plan was to launch his steam powered rocket to some low-ish altitude then launch a balloon once the rocket reached its apex, and have the balloon take the photos.
There was no reason not to just launch the balloon from the ground, other than for Mad Mike to launch himself on his contraption as part of the ordeal.
The balloon and photos were just the hook to get money to fund his rocket escapade from the flat earth folks.