Why can't the gap be settable?
The "gap" is an artifact introduced during audio compression. All lossy compression schemes like to generate regularly-sized packets of data, often inserting unused data to "round out" the last packet. This artifact is rendered by the player as silence.
In an album that has each individual track fading to silence anyway, it's not noticeable. However, an album that has continuous sound from one track to another will exhibit the silence artifacts, and in c-Row's case, it will make the dance mix go off-beat, ruining the "continuous" feel of the mix.
What gapless playback does is create an insanely-quick crossfade, using the information iTunes encodes into the file, to start the audible part of the next song the instant the unused data is reached.
Answering your question another way: I don't see why Apple wouldn't introduce a setting that allows for a 1-second pause between tracks, if enough users let them know.