That would mean that there were plenty of mammoths around just underground, easy to find and within easy reach. If that is the case, wouldn't scientists flock to the area and collect them by the dozens?
I think you underestimates the difficulty to travel across Siberia: almost no roads, no trains, few and scattered inhabitants, thousands of km of steppe in all directions... Huge parts of the country are basically only accessible by helicopter (which is extremely expensive). In fact they even used an heavy helicopter to bring back a whole mammoth to Moscow a few years ago.