as noted above, Superman originally jumped really high (the way the Hulk does). The phrase “able to leap tall buildings at a single bound” (used originally in the cartoons and later in the 1950s TV show) somehow doesn’t sound very impressive if you can fly. The justification, again as noted above, was that Krypton was supposed to be more massive than Earth. Seigel and Schuster were thought to be heavily influenced by Philip Wylie’s Gladiator (where a super-powerful human can jump like that) and John Campbell’s “Aarn Munroe” stories (which used the heavy-gravity-planet explanation), so it seems reasonable.
But, as, I think, Jim Steranko argued in his “History of Comics”, although it looked pretty nifty in the comics, it looked kinda dumb in the cartoons, where Superman looked like an anthropomorphic kangaroo. Actually, I think he looks pretty graceful and balletic in the earliest cartoons, where he’s more jumping than flying. But, perhaps through the influence of those cartoons, which were already uin production, they started changing things in the comics (at least The Superman Encyclopedia says so), so that Supes started hovering, something you can’t do if all your ability is is jumping. Then he started changing direction and downright flying. (The Superman radio show probably played a part in this, too, but i never heard any of the episodes. But it’s easy to suggest flying with a “whistling wind” sound effect, and jumping is a bit more difficult)
In any event, the simple answer is that it’s a helluva lot simpler, in terms of depiction and the lack of detailed explanation required, to simply have your character fly than to try to explain it all as a series of complicated jumps. So Superman flew. No need to explain how or why, and, for the most part, the comics/radio/cartoons/serials/TV shows didn’t even try. It’s all basically wish fulfillment, anyway.
Me, I’ve always wondered about Superman’s combined x-ray vision at heat vision. Someone gave a lecture at MIT once about “Seeing the Universe with Superman’s eyes – seeing the X-ray world”, and it got me wondering about how it really worked. Superman always acts as if he can “turn off” his x-ray vision (and heat vision), but that makes about as much sense as me being able to turn off my “color vision” by will alone. More likely, Supes walks around seeing everyone as wa;lking skeletons and the like. Seeing Lois’ panties? hah! If only he were that lucky!