It isn't just about becoming more perfect themselves, the Borg are trying to help everyone they can achieve perfection by becoming Borg. Locutus and the queen both express surprise and confusion as to why humans aren't just letting assimilation happen. Locutus says "why do you resist us; we only wish to improve quality of life."

This wasn't always the case; in their first appearance the Borg only wanted technological material. Assimilating every person would have been very inefficient for that goal and in fact we see their methods were different in the pre-assimilate everyone days. The Borg are hinted at in The Neutral Zone (before the Borg were written, but clearly they were foreshadowing some big future adversary and the pattern fits even though the distance doesn't) when Worf says "The outpost was not just destroyed, it's as though some great force just scooped it off the face of the planet." Nearly directly echoed in Q Who, first appearance of the Borg, when again Worf says "It is as though some great force just scooped all the machine elements off the face of the planet." Q also makes some remarks about the Borg not being interested in the ship's life forms, only the technology and what they can use and consume. At that time the more efficient hoovering up everything they need method was used.

Clearly at some point Borg priorities changed for some reason. Maybe they consumed the technology for biological assimilation, decided to test it out, and the people they tested it on were radical altruists and that ethos got muddled in the Borg desire for unity. By Best of Both Worlds we see that the Borg have babies; it looked to me like they were implied to be born / grown from Borg but it's possible those were already what would become maturation chambers by Voyager's time. If they are Borg babies perhaps they have some kind of pro-natalist position too: if they can make life perfect then they have a duty to create life in order for life to experience perfection.