> Why do men who have a partner already feel obliged to continue capturing more and more resources?

Two reasons.

First, women are hypergamous and will always be looking for the best mate possible. When men become complacent, they lose their women. The battle isn't over when the first mate is attracted, it has only just begun.

Second, men are polygamous. Men still want to be attractive to other women as well as their own. There is essentially no limit to how many women a man can mate with.

These instincts do rule us, unfortunately. Although many like to think otherwise.