If you compare the demand for websites vs mobile apps, we have about a total of 5 million downloadable apps and about 644 million active websites according to netcraft, which can be interpreted as 131 web devs for every 1 app dev. The pool for webdevs should therefore be much larger.

As for quality, correctly accessing a devs level of competence can be very tricky, regardless of the required skill-set. With web devs, its even more tricky because an "over-qualified" desktop/app devs can do just as much damage in terms of technical debt as a poor quality web dev. A webdev is also a specialized developer, but due to the low entry barrier, many don't realize this.