H1B applicants have to be paid prevailing wage

In theory, yes. In practice, research has shown otherwise: http://www.workpermit.com/news/research-finds-us-h1b-visa-ho...

"We can find an American for this job!" What salary are you offering? "$60k, that's the prevailing wage."

Then there must be a shortage, here's your H1.

If that "prevailing wage" were $100k, there wouldn't be a shortage.

Prevailing wages for the job title they are hired for. What they do is classify them as "junior programmer" and have them replace a senior programmer.