The point is there are a lot of people in this position.
Even if there are, that's not going to lower the unemployment rate significantly; it's more likely to raise it. Say there are 100 million people in the labor force, of which 4 million don't have jobs. The unemployment rate is 4% (4/100). Now 10 million previously retired people suddenly realize that they need to go back to work, and they all get hired instantly. The unemployment rate falls to 4/110=3.6%, not a huge change. And in reality all of those people won't get jobs immediately, and while they're looking they're counted as unemployed. If 1 million out of the 10 million former retirees haven't found jobs yet, the unemployment rate rises to 5/110=4.5%.
Even if there are, that's not going to lower the unemployment rate significantly; it's more likely to raise it. Say there are 100 million people in the labor force, of which 4 million don't have jobs. The unemployment rate is 4% (4/100). Now 10 million previously retired people suddenly realize that they need to go back to work, and they all get hired instantly. The unemployment rate falls to 4/110=3.6%, not a huge change. And in reality all of those people won't get jobs immediately, and while they're looking they're counted as unemployed. If 1 million out of the 10 million former retirees haven't found jobs yet, the unemployment rate rises to 5/110=4.5%.