When you take pictures with a wide angle you "get more stuff" in your photo, so its better value for your money
. Those expensive telephotos only give you the little bit in the centre of what you're shooting.
Cheaper alternatives are manual focus primes even resonably fast (f2.8) 24mm or 28mm of which there are millions of out there. AF is not so important with wide angle as the depth of focus is much deeper. Often you can just set f8 and shoot hyperfocal for anything more than a few metres away.
Also its generally not that important to have fast and huge lenses for wide angle as it is easier to hand hold shorter FL at slower shutter speeds (eg M 20mm f4). Unless you're shooting the ceiling of a dark church I couldn't think of any reason why Sigma could sell anyone a 20mm f1.8.
If you want wider than 20 or 24mm in old lenses they get expensive beacause all of the old glass is full-frame compatible, and larger elements because of this. It is also much rarer because the wide angle on APS size is the FOV of ultra wide angle on the full frame size. See how huge the K 15mm f3.5 is compared to a DA15.
Lastly there is always the 18-55 kit lens or 16-45 f4 both of which aren't too expensive, are AF and you can correct the distortion easily.