Science of prediction is still not mature. Give a country that has been able to give a warning before an eruption? Japan? US? Indonesia? New Zealand?
True. But with the right instruments and proper monitoring, it can be done.
An example?
Right in our own backyard. The US military evacuated Subic and Clark just a few days before Pinatubo erupted. They knew but Phivolcs, if I remember correctly, pooh poohed them.