Companies hire people to make things or provide services if the company believes it can sell those products or services. If, for example, a company is making 100 widgets a day and selling only 50 widgets a day, the company will not hire new employees at any salary, even a low one, because it already has more widgets than it can sell. If a window-washing company has 6 window washers, and enough work to keep only 3 of them busy, it will not hire a 7th window washer, even at a low salary, because he'll just join the other 3 guys that are already sitting around the office doing nothing. In fact, if that state of affairs is projected to last for a while and laws don't prevent it, he'll fire 3 of the window-washers he already has. As soon as people start wanting window-washing and there is too much work for 6 window-washers, a 7th will be hired.
Unemployment is thus driven by demand for goods and services. The supply of workers is immaterial. Companies don't hire extra workers just because they're cheap.
The U.S. has high unemployment across all sectors. There are no sectors that are booming. Thus, retraining yourself doesn't have any immediately useful benefits during the current recession. ALL fields are slumping.