Because 16-35mm is a far, far more useful zoom range that 11/12-24mm. The fact that it doesn't dovetail exactly with other lenses is of zero relevance to photography.
I agree. While it is nice to have complementing focal ranges, and one could assume narrower ranges to have more uniform quality, there are practical benefits to have overlap near/around often used focal lengths. A 16-35mm range covers many practical shooting scenarios. The 11-24mm range is typically tailored to the needs of those who require those wide angles of view, or seek to exploit the seemingly distorted view (comes from viewing from too far away rather than real distortion), thus emphasizing foreground features that are close to the camera.
Besides, the difference between 11mm and 14mm focal length is huge(!), it changes the horizontal FOV from 104 to 117 degrees (that's 32.6 metres width instead of 25.61 metres or 27.3% more width at 10 metres distance). Even the difference between 11mm and 12mm gives a 9.1% wider FOV at a given distance. It is also
a lot more challenging to achieve good image quality for shorter (e.g. 14 instead of 16mm) focal lengths, and lenses become much larger/heavier/expensive.
Cheers,
Bart