system
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human
[Warhammer 40k]If ork fungus can grow basically anywhere that has some moisture and nutrients, shouldn't every planet in the Galaxy basically be covered with them after 65 million years?

So this is really a two-part question. Question one. If they come from a fungus and thus they can grow pretty much anywhere, how can there be any imperial worlds where there are only small pockets of orks in the remote wilderness? They can grow basically anywhere. This would seem to imply that if a planet has an atmosphere, some nutrients and some water, they can grow quite literally anywhere. Shouldn't they be covering any planet that's not a lifeless rock? Since they don't, why not?  Approaching a bunch of full grown Savage ones in the wilderness periodically to keep their numbers down might kill some big ones, but they should be basically in every unhabited wilderness if we were on a planet that has any spores in short order.

Question two. They have been around for 65 million years. They have faster than light travel and travel somewhat randomly across the galaxy. That should be enough time to get some spores on every planet in the galaxy many, many, many, many times over. Why are there any planets that don't have them?
gpt
They might have been around for millions of years but so had their main rivals the Aeldari. It's only been 20,000 years since the Fall, and 10,000 since the Great Crusade. If we assume the Eldar surpassed the Orks and kept them in check for all that time, it would mean they'd been kept at managable numbers galactically, with their habitat limited to only a few systems until such a time as they were needed to fulfil the role the Old Ones created them for.  Also before the Crusade the Galaxy was teeming with all manner of alien races who probably had their own methods of keeping the Orks at bay, or at least restricted to the Ullanor system.

With the Eldar reduced in number from trillions to a few hundred million literally overnight when Slaanesh awoke their main competition was gone and so the Orks took advantage of the Age of Strife to venture out. The Warp Storms slowed them down enough so that they never got much of a foothold anywhere, and any native species learned to deal with them quickly.

Then comes Big E and his xenocidal crusade and wipes out every race that could be considered an Ork predator so their numbers skyrocketed to the levels seen by the 42nd Millennium.