Because it never completely decays. Every 0.07 seconds, half of the
remaining isotope will have decayed. Think of it like this:
You have to run 100m. You're not very fit, so you have to rest every once in a while. Let's say you take a rest when you've gone half of the remaining distance. So at first you run 50m, then 25m, then 12.5m, then 6.25m etc.
Will you ever actually reach the finish line? Well, theoretically, no. Eventually the measurements will be so small that simply breathing may move your shoe far enough to cross the finish line, but it's just an analogy
If you're looking for a more detailed mathematical explanation let me know.