For several reasons:

First, there is always risk in mining so that is why an NPV calcualtion is done. Having a 40 year resource really does not do much past the 20 year mark as the discount rate nullifies it. Laws chnage, governments change, a native issue can crop up, taxes change, sales prices chnage, mining costs change etc.

Second, it is a giant leap for junior explorers to move into mining. Geologists are not mine runners. Finding a resource is difficult enough, finding an economic one is even harder. Extracting it and making money doing so is a different ballgame.

Third, you have the shareholders. If someone made a public offer today for $1B that would be about $17 per share. Even if the Board recommneded to not sell, the vast majority would sell given the gains and really how could one not? Give me that offer today I am gonzo.

A Billion dollar offer would be difficult to refuse. It would probably be close to insanity to do so.