Saying "programmers can't" ignores inherit selection bias in the people coming to your job interview.

To be able to draw conclusions about the programming populace in general, you'd have to find a way to randomly sample programmers regardless of whether they were unemployed, where they worked, whether they worked for themselves, and so on.

The Lemon Market law applies here. Here's a write-up for the lemon market as it applies to programming languages: http://outspeaking.com/words-of-technology/the-lemon-market-of-programming-language-adoption.html

Floods of people selling low quality goods causes the median value to plummet (people adapt jobs, interviews, and pay scales to lower qualified applicants) which undervalues qualified applicants.

Perl is a good example. A lot of jobs were eliminated over the past several years as PHP, Python, Ruby, and then other things, became preferred technologies. A long list of companies are now reporting difficulties hiring qualified Perl programmers.

That may be counter intuitive. If lots of qualified, experienced, senior level Perl programmers were laid off, you'd think that they would be easy to find, but it's more complex than that:

Smart programmers don't compete over rare job openings in their same specialty, especially if those rare openings would be a demotion. They retool. I can name several people who spoke at conferences, wrote books, and were well known in the community who now work in another technology.

Companies whine too, instead of doing the needful. I talk to companies all of the time who claim to be desperate for qualified help, but they still set up endless barriers for themselves: hiring right now and then not hiring anyone no matter how good they are until "a position opens"; paying a fraction of what the same applicant would make using another of their skills; being closed to telecommuters no matter how much butt they kick; extremely narrow view of what a programmer can accomplish; unwillingness to touch mid level programmers who show a lot of promise and insisting on only hiring senior talent. I feel like I hear more complaining from companies than programmers. If you're not buying people away from Google (and Google swallowed up a lot of top Perl talent, to make them do network engineer and sysadmin stuff), then find a way to compromise and create incentives that way.

Really good programmers also know that very few companies value talent, and I mean that in a specific way. Perhaps the small companies really want good programmers, but without an understanding of the nature of programming ("programming is dreaming" or something similar), they're going to routinely hire the wrong people, and then mismanage them. They'll value a facade of professionalism over life long technical passion. This applies to small companies and large companies. State Farm is opening up mega offices here in Tempe and hiring like nuts, but it takes hours to input all of your stuff into their buggy as hell job portal, and then off-shore non-technical help from Indian pre-evaluates the score assigned to your application by a computer program. This is almost perfectly designed to keep good talent out. Small companies, stereotypically, are running ASP.NET or PHP with management and programmers who have a very, shall we say, minimal take on what a programmer's skill set should be. This makes applying for jobs an enormous waste of time, if you're qualified.

Project management know how is critical too. The experience necessary to make the right decisions not to sabotage the programming staff is rare. Even if a random company actually did know how to identify good talent, that good talent may not want to work there, and vetting companies is extremely time consuming.

This creates a lot of incentive to just go work for a large company that knows how to hire, and knows how to manage, if you are a good programmer and willing to relocate. This means Netflix, Amazon, The GOOG, and a handful of others. Formerly, this was Microsoft, then IBM, then Rand, Cray, Digital, Xerox, Bell Labs, and so on. No tech company comes crashing into market prominence without (perhaps accidentally) getting really good programmers, turning them loose, and providing resources for them. The oral history of SNOBOL (nothing like COBOL despite the pun) illustrates the culture at Bell Labs that made so much innovation possible: http://amhistory.si.edu/archives/AC0196_gris720521.pdf

Because of the difficulty of finding a working environment that's empowering and supports rather than sabotages productivity, few good programmers will leave a workable gig. They tend to stay put. Rather than accepting applications, you have to hunt them out, and then make a sale. There was a company that matched up productive open source programmers (measured in terms of github contributions) with local opportunities, but guess what, that's not how companies wanted to hire. Perhaps it's ego, but they wanted brilliant applicants to notice the opening on their web site and then call a phone number. If I were hiring, I'd be on github committers like ugly on an ape.

Qualified programmers also have the option of going in to management, working with consulting firms as independents doing short term contracts for large amounts of money (with nice breaks between), or starting their own consulting business.

This isn't snottiness on the part of good programmers; often they hide away from the politics, fashions, and industry BS (take Why the Lucky Stiff as an example; indications are that he worked for a small town Web shop doing VBA while being the best known Ruby celebrity in his spare time). They hide from money too if it means they can be productive and stable and stress free.

Yes, programmers should constantly be improving their skills, but that won't actually help you get programmers who do.

tl;dr: the first two paragraphs, and the last.