Answer is simple - desire of individuals to better themselves and their families.
If you take a look back at any period where humans achieved a massive breakthrough in production and productivity that improved the entire human civilization, it was usually a side effect of them wanting to better them selves somehow. I mean individuals, not entire groups necessarily.
For example when colonies in Americas were set up people wanted to exploit the resources there for their own benefit in order to generate wealth. They went to the colonies themselves as incomes were better then what they could earn at home. Pioneers borrowed coin and promised great returns on investments. Remember that it is usually not the case that 1 mega-investor gave most of the capital and expected a return - most probable case is that many, many people would put some of their savings in the hands of the few to try and gain some return on that small investment. With that money, the pioneers could start to colonize, build, invest and produce while the initial borrowers could buy something nice once their return on investment came back. Beneficial for everyone.
Infrastructure is being built for exploitation of natural resources in order to profit from them. However, creating that infrastructure needs more workers. Which required more resources like food, clothing, tools, housing, which required more infrastructure... it is a snowball effect where wealth attracts wealth. Countless people see that they could sell their services in this new sector better so they move and try to make a better living. With more wealth and territory human populations explode in size, which in some years ends up requiring even more resources to be acquired etc.
While some people notice the growth and wealth being generated are naturally attracted as it is a whole new market with very little competition and a relatively easy entry, others simply want a new adventure, a life away from their old family (they can marry someone the family does not approve of perhaps?). They like risk, they follow their friend how got them an amazing position in some excavation company etc.
This snowballing effect would go on until humans reach some sort of equilibrium - extraction of additional resources is no longer straightforward and entry in to the industry becomes difficult. Human populations grow rapidly but are eventually forced to slow down their growth once easy wealth-generation spots are filled in. Governments slowly start putting in regulations to prevent injury or dangerous activity. Additional bureaucracy, that attempts to achieve good, slows down human activity as it puts a heavy cost of entry (licences, training, taxes etc).
But guess what - a new colony is just being made and life is a bit easier there and money can be made so much easier. You work the same hours but you get like 3 times more money because they are short on manpower... and the cycle repeats it self.
Only limit is technology and government.