Why did mercantilism require European nations to colonize new areas?

Mercantilism was an economic theory in Europe that argued that a country's worth was based on its wealth. So the countries needed to gain more and more wealth continually so their country continued relevant. Because of that economy had no other way to turn other than try to discover new places with new goods, with new ground to plant on and new things to sell and make more and more money. This was only made possible when countries started to colonize new areas such as the Americas - Spain, Portugal, and England. Colonies later became a good source of income, usually, countries would make the colonies pay high taxes with no or almost no investment.