Why are any radioactive elements left?

Well, it is called “half life” for a reason. The amount of a radioactive element is cut in half over a specified amount of time. And then it gets cut in half again, and again, and again. Mathematically, it would never reach zero.
Plus the fact that the product of the decay might be a radioactive element itself, with its own half-life.

@ragingloli Then logically there must have been plenty of radioactive elements for them to last 4 billion years, from the last nova.

I wondered if the half-life doesn’t take effect until mined? I know it sounds weird.

The half-life of uranium is 4.5 billion years, so yes, when the planet was formed, there was twice as much uranium inside of it as there is now.
In fact, because uranium decays into various other radioactive elements, which themselves decay into radioactive elements, and so forth, until the last step decays into lead, the amount of various radioactive elements other than uranium might have actually increased over time.

There’s lots of novas and supernovas. In fact, an extragalactic novae, M31N 2008–12a, erupts as frequently as once every 12 months.