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.

It serves a double purpose of eliminating competition. They'd rather not share resources, negotiate treaties, or justify their actions. They have a singular focus, and anything getting in the way of that must be eliminated. Also, because they're a great villain for the stories as they've been written.

They aren't driven by a want of technology. They're on a quest for perfection (and "technological perfection" is only one facet of that.) Even if only one single entity is left standing against them, that would be a thorn in their side reminding the hive mind they weren't there yet. They hadn't assimilated every single being in the cosmos.

Also, they see assimilation as a kindness to the assimilated individual in a twisted sort of way. Maybe even a moral imperative. There are a lot of places where the borg go on about how "you'll never be alone again if you just let us assimilate you."

And, honestly, by "they" I kindof largely mean "she". (Pronouns and sci-fi! Happy June, people!)

I guess in a way it’s like a virus. They conquer civilizations. Catalog their technology and biology and use their industry to produce more borg to continue the process.

It seems highly inefficient to me.

Exactly. It's meant to be an obvious piece of hypocrisy.

The real reason is that the Borg are so thoroughly sociopathic that they don't care about their impact on other's cultures, or even about the unnecessary costs to their own colony members.

I’d always understood Borg assimilation to mean conquest as a primary goal and any technology obtained in the process was just icing on the cake.

Rather like the Draka, except without the extreme racism and absurd plot armor that would make even the Lannisters blush.

The Borg are wise enough to know culture only exists in individuals, so you can’t get the whole culture unless you get all the individuals.