Because they have 'em! Halos don't appear all the time, and when they do, they may not be visible to every single person around. But saints are filled with the Uncreated Light and from time to time, not all that infrequently, it shines forth from them.
St. John the Wonderworker of Shanghai and San Francisco (I LOVE to say that; it sounds so incongruous!) lived from 1896 to 1966. He was glorified nearly 30 years later, in 1994. In preparation for his glorification, his grave was exhumed and his unembalmed body was found to be incorrupt and exuding a sweet fragrance.
This picture of the saint is from the website of a parish named for St. John. I found that website via my new godson's blog. St. John appears to be reading the Gospel.
Here is an icon of St. John, who also served Washington, D.C. Here he is shown holding a replica of the Cathedral of St. John and Baptist, which he once served, and in which church I was baptized, chrismated, first communed, and married to Demetrios.
This icon is also the only one I know of that depicts automobiles. I've forgotten the story, but he disrupted Washington traffic for a while (on Dupont Circle, I beleive) in protest of something or other, and this is what is being commemorated in that scene at the bottom of the icon.