Why? Seriously...what is the practical difference between someone not trying to work because they don't think they'll get a job, and someone not trying to work because they don't or can't work? Who is more likely to get a job?
It's a question of what you're trying to measure. The point of measuring unemployment is to see how many people who could be working...who could have been hired in a particular month...are not working. People not looking for work could not have been hired...could not have been working.
Analogy time....if you're a retailer and you sell out of a particular item. You sold 150. You find out that 180 people came to your store to buy it. And you find out that 20 people called the store, or found out from someone who called or visited, that you were out and so didn't show up. And you find out there were 10 more people who say they wanted it, but didn't check to see if you had any either by contacting you or a friend.
How many could you have sold if you had had enough inventory? Obviously the 150 you did sell. Obviously the 30 more from the people who showed up and walked away empty handed. We can also assume the 20 people who called or asked would have showed up and bought if they knew there was inventory. So that's 200 we know, or are fairly certain you could have sold.
But what about the 10 people who didn't show or check. Can you say that they would have bought if there was enough inventory? No. Regardless of your inventory, they wouldn't have shown up anyway, even though they say they wanted it.
Now, it's useful to know that there are 10 more people who might buy the item sometime in the future, but you can't say that for the particular day in question that you could have sold 210, but you can say you could have sold 200.
Does that make more sense? That "but they still didn't get the item they wanted" doesn't mean they would have gotten it if you had enough.