Because they can see it from surface and can bulk sample and do...

Because they can see it from surface and can bulk sample and do trenching,
Following the Mineralisation etc. could be one reason.

Another may be that drilling is also a bit needle in a haystack,
While it is good for determining the depth of Mineralisation,
I think trenching and Bulk Sampling gives a better indication
of what's there tonne for tonne....IMO....

It's not like SUM has to drill to 50 or hundreds of meters to prove whats there,
And often Drilling is to work out if it's viable to dig a pit that deep in the first place.

In this case valuable material can be processed from the start....