Once in a while aspiring people aim to drill through the Earth's crust to reach the mantle, but why do they want that when the mantle is actually on or very close to the surface in volcano craters? Can't scientists just use volcano craters for researching the mantle rather than drilling miles and miles through the surface elsewhere?
why don't they just use volcano craters?
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.
I am not a geologist, but my understanding is as follows: volcanoes may be felsic or mafic. The latter type gets its 'fuel' from hotspots from within the mantle. These volcanoes are located at plate boundaries, where the crust is thinner. If the question is 'why can't we send a probe down such a hotspot volcano into the mantle' - I'm not sure why that'd be easier than just drilling into the oceanic crust, where the mantle is a few km's beneath. After all, oil wells are drilled far deeper than that (though they tend to be located near the continental shelf where there's an accretionary wedge of mesozoic detritus).