Cave systems can be quite large. It could have taken 80 years to get through enough of the cave. And then you have to consider it's a variation of the halting problem. Until you find something, you can't know if you're actually going to find something or nothing. So do you keep digging in this spot or do you move to another spot?

So while they already found remains in this cave, the new remains probably weren't in the immediate area.

5 days, 5 years, 50 years, none of those timeframes would be surprising. It all tracks.