Because the rocks extruded in a volcano suffer a lot of changes during its way up. Moreover, a mantle rock is composed by many different minerals. When temperature goes up, some minerals melt first. So, you expect the extruded magmas to be enriched in those minerals. The extruded rocks are not sampling the mantle properly. Of course they provide tons of information, but is not the same as having a fresh mantle rock.

Also the possibility of finding new life forms. We've found life drilling several miles down. How much further does it go?

I'd guess for purity reasons. Volcanic eruptions don't always come from the mantle - in fact think convection cells, the rising material actually comes from deep down

Because we want to know what's down there, not what comes up here.

Without adequate safeguards, what's down there will come up here (at least to the top of the borehole), and create a new 6km volcano. I'd name it Moria.

I don't think it works like that. If you are in the middle of an ice burg and drill down to the ocean below it, does water start shooting up through the hole?

Yes, if the bottom of the iceberg-hole is below sea level.

If the mantle at the bottom of this new bore-hole is under pressure and liquid, then it will go up the bore hole until the pressure above it equals the pressure below. If the bottom of the bore-hole hits a part of a liquid mantle with a lot of pressure, then we might get a volcano. Hence, Moria.

The mantle isn't liquid, it's solid rock.

So, If you want to create your own island in the middle of nowhere all you have to do is drill a couple of holes to the mantle. I can certainly see some people creating islands this way in the hope of using them decades latter.

Superman movie.

If they hit adamantine, I say we call the whole thing.

Yeah, it would be bad if any of the clowns got out.