Slop.

Quoting from A Few Notes on the Culture

What they're trying to get at:

The attraction of Orbitals is their matter efficiency. For one planet the size of Earth (population 6 billion at the moment; mass 6x10^24 kg), it would be possible, using the same amount of matter, to build 1,500 full orbitals, each one boasting a surface area twenty times that of Earth and eventually holding a maximum population of perhaps 50 billion people (the Culture would regard Earth at present as over-crowded by a factor of about two, though it would consider the land-to-water ratio about right). Not, of course, that the Culture would do anything as delinquent as actually deconstructing a planet to make Orbitals; simply removing the sort of wandering debris (for example comets and asteroids) which the average solar system comes equipped with and which would threaten such an artificial world's integrity through collision almost always in itself provides sufficient material for the construction of at least one full Orbital (a trade-off whose conservatory elegance is almost blissfully appealing to the average Mind), while interstellar matter in the form of dust clouds, brown dwarfs and the like provides more distant mining sites from which the amount of mass required for several complete Orbitals may be removed with negligible effect.

What they're failing to understand:

By Goîdel out of Chaos, the galaxy is, in other words, an immensely, intrinsically, and inexhaustibly interesting place; an intellectual playground for machines that know everything except fear and what lies hidden within the next uncharted stellar system.

This is where I think one has to ask why any AI civilisation - and probably any sophisticated culture at all - would want to spread itself everywhere in the galaxy (or the universe, for that matter). It would be perfectly possible to build a Von Neumann machine that would build copies of itself and eventually, unless stopped, turn the universe into nothing but those self-copies, but the question does arise; why? What is the point? To put it in what we might still regard as frivolous terms but which the Culture would have the wisdom to take perfectly seriously, where is the fun in that?

Interest - the delight in experience, in understanding - comes from the unknown; understanding is a process as well as a state, denoting the shift from the unknown to the known, from the random to the ordered... a universe where everything is already understood perfectly and where uniformity has replaced diversity, would, I'd contend, be anathema to any self-respecting AI.

Probably only humans find the idea of Von Neumann machines frightening, because we half-understand - and even partially relate to - the obsessiveness of the ethos such constructs embody. An AI would think the idea mad, ludicrous and - perhaps most damning of all - boring.

This is not to say that the odd Von-Neumann-machine event doesn't crop up in the galaxy every now and again (probably by accident rather than design), but something so rampantly monomaniac is unlikely to last long pitched against beings possessed of a more rounded wit, and which really only want to alter the Von Neumann machine's software a bit and make friends...