It'd likely to be more accurate to say that some are left that simply don't explode yet. A good example would be certain cases of cluster munitions that are designed to hit airfields. A most of the explosives will go off and crater the concrete and asphalt, but others will remain unexploded and sensitive to detonation. As runway repair work cannot commence until those unexploded munitions are cleared, the airfield is out of action longer than it would be otherwise.
I understand in these cases and other ones such as scattering mines that a timer can be set so the mine or munition deactivates after a set time (say, a week) by deactivating the fuse.
Not all are designed in this way of course. And apologies, I'm looking for source links, but I can't find them at the moment.
Edit: Here's an example https://en.wikipedia.org/wiki/GATOR_mine_system
Given the sometimes impressive fraction of duds among bombs, I am not sure I trust a mine that supposedly deactivated itself on a timer. It may work 9 times out of ten. But it doesn't really make a big difference if it's the first, second or the tenth mine that gets you and rips off your leg.
They deactivate by discharging their power supply, after which they are lacking the stored electrical energy needed to trigger the electrical detonator.
It's not fool-proof, but a lot better than chemical/mechanical mechanisms that try to go inert.
I understand in these cases and other ones such as scattering mines that a timer can be set so the mine or munition deactivates after a set time (say, a week) by deactivating the fuse.
Not all are designed in this way of course. And apologies, I'm looking for source links, but I can't find them at the moment. Edit: Here's an example https://en.wikipedia.org/wiki/GATOR_mine_system