Why is lead found in all deposits of uranium ores?

Radioactive uranium decays in a series of steps. Lead is the stable element at the end. So even if the ore originally contained no lead, the radioactive uranium isotopes in the ore will decay and become lead over time. Granted it takes a long time to do that, but it will eventually become lead. That's why uranium ores contain lead. It's the left over after the radioactive uranium isotopes have completed their decay.