no vulcanoes aren’t a location with thinner crust, but a place where a bubble of magma is stuck in the crust, because of a hotspot or because of subduction or other reasons. When the pressure is too high, or there are forces that push the magma upwards, it “leaks” some magma, some times violently, some times no, like in Hawaii. An inactive volcano probably has a bubble without enough pressure to release it, and that has no upward forces nor a way of building more pressure.