An extraction method needs to balance extraction recovery...

An extraction method needs to balance extraction recovery efficiency vs. extraction costs per tonne whereas the assay method needs to be very accurate.

Extraction methods are a partial recovery, so they can recover 50, 80, 95% of the metals while assaying should recover ~100% of the metal.

So if we can recover 98% of the metals relatively cheaply at a high daily tonnage throughput it might not be worth spending the extra time and money to recover the remaining 2%.

The dirt containing the 2% not recovered is called the tailings and can get processed using a secondary process e.g. heap leach at a later date when there's enough tonnes stockpiled to make it worthwhile.