But then why did people keep conquering and colonizing new lands?

That is a very good question on which books have been written. Some of this was about religion and prestige, and competition with others. Some of it was various sovereigns being convinced to fund dubious (in retrospect) ventures by good marketing.

We have our biases and our cultural zeitgeist, and folks in the past had theirs. After the Otman Turks conquered Constantinople and killed off the Roman empire for good, the Portuguise started looking for an alternative route to do spice trading (and also look for Prester John, the mythical Christian king in the east). "We are looking for spices and Christians" was the motto.

The English had complicated reasons to start colonizing that were not all about money. A lot of the times it felt like colonial things happened for complex reasons (e.g. having to do w/ what was happening w/ Christianity at the time), and the Crown tried to find ways to make money off it.

It was the case that at some point the sugar trade became very valuable (e.g. to Napoleon the tiny sugar-producing possessions of France were worth much more than the entirety of Louisiana), but this happened much later -- there wasn't a "master imperialist plan" at all.