There's more than 8 000 000 000 of us, if everybody were just doing the thing with the greatest priority it would be very ineffective division of work. Like digging one hole with 1000 people.

As a humanity we can (and should) multitask, especially when it comes to science and inventions, because you never know which seemingly pointless path can lead you to a great breakthrough.

Imagine everybody since antiquity worked only on the greatest humanity problems as they've seen it at the time. No useless stuff like music, so no irrational numbers for example :)