To give this a slightly fuller treatment than my (admittedly rather snide) earlier remark:
First, you don’t. You get it under the messy rules of the Common Volumetric Accord, which is a compromise hashed together between a whole bunch of civilizations with very different ideas about how legal systems ought to work (ranging, in this case, from consensualism and right of first appropriation, through state sovereignty, through acquiris quodcumquae rapis (as Pratchett put it), to “If God gave it to us, it’s ours.”). Doctrine of the Ecumenical Throne notwithstanding, Imperial-issue law and ethics doesn’t hold sway across the entire galaxy. Yet.
This could itself be used as a selling point for the scheme, incidentally.
Second, colonial corporations are very, very big organizations, and capable of installing the requisite planet-sized infrastructure that colonists tend to want. And if you can homestead sovereign and infrastructural rights to a whole planet, you’re more than welcome to. (In sensible legal systems, acquiring those doesn’t get you all the rights to all the real estate on the planet, mind, because property rights are bundles and that would be quite the overreaching claim, but still.)
As a tangential aside, how does homesteading on this scale work under these “sensible legal systems” (presumably including the Empire’s, I’m assuming)? I’m aware of the idea of the “bundle of rights,” but I was under the impression that if you’re taking something out of terra nullius in an act of original appropriation, it’s an all-or-nothing proposition – either you own a particular unit of owned and/or enclosed volume outright and thus can do whatever you wish with it, or you don’t and you can’t. One could make the argument that anything on a sufficiently large scale as to provide a planetwide benefit so as to raise its value to would-be inhabitants (such as full-body atmohydrosphere ecopoesis) constitutes an “improvement” that gives claim to the whole shebang.
Most colonial corporations have little ambition of becoming interstellar governances, which is a whole other ball game that doesn’t fit their core competencies or add much shareholder value
Who said anything about “governance”? A landlord isn’t a state, are they? 
…Alright, I’m guessing that that isn’t what you’re getting at. Still, given that this is the Associated Worlds we’re dealing with, the lessor could make the lease agreement as restrictive or unrestrictive as they wished; so long as the lessee holds up their end of the bargain and the rent (whatever the amount agreed upon) comes in on time by the end of every pay period, then the lessee should be free to make whatever arrangements they want for any services that the landlord doesn’t provide themselves.
And if the lessee doesn’t hold up their end of the bargain… well, this being the Associated Worlds, I’m guessing distraint is relatively quick and painless (for the landlord, anyway).
Populations don’t generally grow fast enough in developed civilizations to make a giant land-grab profitable anyway, since the number of available colonists is finite
The number of available sites is also finite, and supply is much less elastic at that – especially in the absolute sense, on anything outside of geological time scales.
I’m going to guess that the fact that the Associated Worlds is a “nearly-post-material scarcity” on the whole hasn’t eliminated the advantages that one particular plot of volume may have over another due to location, ready abundance of natural resources, or proximity to other services, and thus that the law of rent ( The Law of Rent, Part 1 ) still applies. As you put it yourself:
This is also how many colonists prefer it, because it’s nice not to have to provide everything yourself.
I’m also guessing that in an economy as robust and developed as you’ve implied there are economies of scale such that even minute differences in per-unit prices may translate to rather large cost differences in the bottom line.
Presumably, anyone who knows what they’re doing could set the terms of their lease such that they find a comfortable midpoint where their lessees would still get more utility out of that particular plot than if they had to do all the surveying, provide all their own materials, etc. themselves, while the lessor still makes a comfortable revenue from the rent collected (if not, strictly speaking, as large an amount up-front in a lump-sum payment). In fact, it need not be all that different in execution from how these planetary development corporations already do business – the only real change being that they still keep their underlying ownership in the volume in question itself. Nor would it be all that different than the land tenure scheme implemented in places in our world such as, say, Hong Kong or Singapore.
After all, it’s not like I’m proposing that the people who would embark on such a venture would literally leave all that space entirely fallow and undeveloped until all the volume in the Universe were already locked up simply to run up the prices – that would be silly and impractical.
You’re going to piss off a lot of people who see you grabbing all the good stuff out from under them, and even if you have a perfect legal right to do so under your own system, it’s unlikely to be worth the trouble, given the above.
To quote something you told me in an earlier conversation when a similar line of questioning came up:
“If you liked it, you should have put a ring^Wtitle on it.”
Is it their fault that someone more motivated to do so got there first and locked them out? Is it their fault that those who might have wanted those particular sites now either have to deal on the new owner’s terms, or else settle for something a little more marginal?
(I will say that it’s not entirely accidental that I’m arguing from the opposite perspective than the tack I was taking in that particular discussion… )
