My personal theory is that the the demand for IT workers in general (devs, QA, "architects", program managers, ops, sysadmins... Etc) is so damn high relative to supply that our industry attracts and retains large quantities of people that are just plain shouldn't be here.

Witness the distressingly high number of devs that fizzbuzz catches out.

I don't know if they are in the wrong industry / career, are just generally incompetent, or could be good and just don't care, but it seems like the percentage of incompetents is rising.