I mean realistically it's because if they did there wouldn't be a show but let's be real here.
Quite possibly if they did there wouldn't be
Borg.
After all, the Borg survive through assimilation. With nobody left to assimilate, survival ends.
1) We have yet to see much sign of Borg creativity. If "free" species cease to be a source of inventions, the Borg stagnate and soon become unable to cope with mundane problems of sustenance.
2) Worse still, we have yet to see much sign of Borg procreation. In "Q Who?", Riker claims to be witnessing some sort of in vitro growth of new Borg when he finds babies in desk drawers. But later episodes establish that the Borg do not give birth to babies by any means - this through witness statements from actual Borg or ex-Borg, who may be lying but who nevertheless would know more than Riker.
By all accounts, they should have assimilated a good portion of the galaxy by now if not the whole thing.
They
could. Should does not necessarily follow. Although for all we know, they have indeed assimilated everybody a couple of times already - but in order to survive, they have then let the galaxy go fallow again, giving rise to new victims. Nobody knows how old the Borg are, although Guinan's minimum estimate of "thousands of centuries" from "Q Who?" is not contradicted by Q, the one character who
could know.
According to TAS "The Slaver Weapon", it's an accepted in-universe fact that all sapient life in the galaxy died out at least once already - a billion years ago in a big war where the eponymous Slavers were gotten rid of - and had to develop anew. And according to TNG "The Chase", it's no wonder sapience bobbed back up again ASAP, as the very chemistry of life has been rigged by ancient intellect to forcibly create sapient bipeds capable of cross-breeding.
They just go to whatever planet they feel like with no tactical strategy in mind.
More probably we are too stupid to comprehend their strategy. VOY "Child's Play" explicitly shows what
TheSublimeGoose mentions above: the Borg attack a planet with relatively weak forces, deliberately allowing it to survive and develop better means of resisting the Borg. The Borg then attack again, harvest the new means, and again let the planet survive, waiting for it to develop yet more and better means they could harvest the next time around.
This could well be what's happening with Earth or the Federation as well. The Borg lose little or nothing by letting the Feds slaughter one Cube or Sphere or whatnot, but gain a lot.
Heck, for all we know, it indeed is "or nothing". From Seven of Nine's experiences with a malfunctioning feature of the Collective in "Infinite Regress", we learn that anybody ever assimilated is immortal. No matter whether his, her or its assimilated body gets phasered to oblivion by cheering Feds, the personality keeps on living inside the Collective, and can even be uploaded to a new body if necessary (or at least this can happen through malfunction, which is suggestive enough).
They attacked with Alpha quadrant despite the fact that there are planets in the delta quadrant all around borg space that haven't been assimilated. Wouldn't it make more sense to just assimilate everyone next to you, acquiring all of it and creating a vast borg empire?
The only indication that the Borg might be interested in conquest comes from ST:First Contact where Picard claims the Borg have been "advancing" and the Federation "falling back". The good captain seems to be describing Borg existence in classic, conventional military terms there (and never mind that what he describes has never been seen on screen). But whether the Borg would hold on to turf "conquered" is unknown. Most of them appear to live in free space, in those vast Unicomplexes of theirs. For all we know, conquest of planets or territory is just part of their strategy of goading primitives into becoming a bit less primitive, so that they would be worthier of assimilation.
The Borg "spreading" is an illusion anyway. They are already everywhere; it's just that, as per VOY "Dark Frontier", they prefer to remain invisible, and succeed in that easily enough. They have clearly been to places, though, and have e.g. assigned relatively low Species numbers to cultures in the vicinity of Earth, supposedly long before they "first" visited the Federation in the first season of TNG (a "first" this VOY episode explicitly proves false, without in any way suggesting that the slightly earlier "first" in that episode would be any less false).
Timo Saloniemi