他们为什么不直接用火山口?< / p >

Because volcano craters don't go to the mantle.

Here's a sketch of how the crust, mantle, and volcanoes look like:

Tan colour is crust, orange is mantle. Your question may come from the misconception that the mantle is molten, but it is actually not. It's solid, except for very localised cases where the mantle is molten (see diagram for examples). This magma then rises through weaknesses or cracks in the solid crust to reach the surface.

Drilling through volcanoes will only give you information on the shallow plumbing of the magmatic system through the crust, and not much on the mantle.

There are places where the mantle is very shallow, for example in oceanic spreading ridges, but those are active magmatic zones and drilling through liquid hot magma is not something that's feasible.

Finally, there are places where mantle rocks have actually been exposed on thee surface, such as ophiolites, but these are probably exposed because the mantle there is slightly unusual. It provides only limited information on the more common and deeply-buried type of mantle that's elsewhere.