If it ain't broke, don't fix it. At least that's the motto of the military forces keeping these machines running.
Yes, there are indeed some operational T-34 tanks in third world countries, but good luck finding them! There are also Spitfires, and three candidates from 1957: the Lockheed U-2, the Lockheed C-130 Hercules and the Boeing KC-135 Stratotanker. But those old units didn't even make it into our top 10.