Because the ash, rock and lava devastate the area surrounding the volcano - and dust & ash can be carried for thousands of miles in the atmosphere.
There has never been an instance where a volcanic eruption has been prevented from occurring by human means.
The deadliest volcanic eruption in history was in 1815 in Tambora, Indonesia. 92000 people died as a result of starvation because of this eruption.
Three people died in the Lassen Peak volcanic eruption in 1915. They were suffocated by toxic gases and ash during the eruption.
people die
Tell them to run...
Billions of people every day manage to struggle through not being in a volcanic eruption. I think you'll get by somehow.
We do not know how to stop earthquakes or volcanic eruptions.
Sometimes a volcanic eruption is not that bad but sometimes it can be very bad. For example, people could die because of it and houses can be destroyed.
They will probably die
about 600
the goverment warns the people to evacuate the area
Around 15,000 people died in the Unzen eruption in 1792, which was caused by a combination of a volcanic eruption and resulting pyroclastic flow.