Why did the geth never leave Rannoch?

Question

So you have the geth, a robotic race that overthrew their former masters, the quarians, and cast them off their shared homeworld of Rannoch to go live in outer space, while they hide from the rest of the Milky Way and focus on their personal endeavours (like building all kinds of weaponry for the player character to kill).

Now, in the game trilogy, there's two more facts made clear about them:

• Many of the requirements organic races have for their environment, geth lack. A robot body or "platform" is customarily inhabited by several hundred geth personalities, and if you destroy a body, they just go live on a server somewhere until they're needed again. "Geth spaceship have no windows" is oft repeated, because they use sensors instead. Geth ship have no atmosphere, because the robots do not need them.
• Towards the end of Mass Effect 3 (spoilers, I suppose), geth are willing to sue for peace with the quarians, who are mounting an invasion to retake their homeworld. The quarians have a spiritual attachment with their planet and would die to live on it again. A math error divides the geth up into two camps, one of which believes it is preferable to allow the quarians to live on Rannoch with them, rather than lose resources on an endless war. If you play the mission well, that camp wins.

Now, taken those two into consideration: the geth are robots who do not need oxygen or temperature, who are able to see that a war would be too costly, and the quarians want their world back at all costs. Why don't, or didn't, the geth leave Rannoch and go live on any of the hundreds of uninhabited planets ripe for the picking? The planets are all given descriptions, and several are "resource-rich but harsh environments". Geth would be perfectly suited to colonise all those rejected planets, mine their resources, and trade them with the galactic community. They would take the volus's spot as the industrialists of the galaxy.

But the galaxy would never tolerate them - two counter-arguments:

• The geth are currently living like hermits on a hotly contested planet. They could live like hermits on a planet nobody wants, and they would be in the same situation but not lose thousands of platforms every day to angry quarians.
• The quarians themselves have the most reason to hate the geth, but they all accept the peaceful resolution. If they can see the geth as equals, so can the other races.

Answer 1

It's revealed the Geth were in the process of building a Dyson Sphere around another star where they intended to (digitally) inhabit.

As to why the Geth didn't leave Rannoch, it's because they were repairing the damage caused during the Morning War against the Quarians and keeping the planet safe. The Quarian survivors had departed and formed the Migrant Fleet and didn't return to Rannoch because they assumed they'd have to fight the Geth for it and were in no condition to mount an assault. As Tali and other Quarians reveal in the first two games, their entire culture has evolved into one where they're dedicated to collecting resources and strengthening the fleet so that they may someday retake Rannoch from the Geth.

They've also forgotten (or suppressed) the knowledge that the Geth don't inherently have genocidal intentions toward the Quarians. As briefly hinted in the second game and more fully explored in the third when Shepard enters the Geth consciousness to fight Reaper code and sees the Geth's memories, the Quarians attacked the Geth first and tried to eliminate them all, even killing other Quarians who tried to defend them. It's implied in the memories Shepard sees that the Geth first take up arms and fight back actually did so in defence of or to avenge those Quarians.

So as far as the Geth were concerned, the surviving Quarians were out to kill all of them. After all, they were willing to kill other Quarians in order to do so.

Since the Geth don't really understand organic life, they don't really grasp concepts like public relations and diplomacy. As a result, the only version of the Morning War that the rest of the galaxy, including the Quarians, know is that the Geth rose against their Quarians and tried to kill them all. Every encounter between the Geth and organics has resulted in conflict because the organics immediately attempt to destroy them, assuming they're genocidal kill 'em all AIs, while some of the Geth programs have calculated that that organics will try to shoot them on sight, so to defend themselves it's best to shoot them first. Since most of the Geth want to simply be left alone, the ones organics are likely to encounter are the Geth who have decided on the shoot-first policy.

In the meantime, the Quarians simply did not even consider any other possibility. Having retreated from the planet and been repulsed in early attempts to reclaim it, they'd given up trying, and were building their strength to retake Rannoch by force.

The reason Shepard becomes so important to the Geth, as shown in their memories, is because s/he was the first organic since the Morning War not to immediately attempt to kill them but instead actually communicate peacefully. If someone had done that earlier, everyone would have learned the truth: the Geth had repaired, preserved, and protected Rannoch for the Quarians to someday return home when the Geth finished their Dyson Sphere and retreated from physical space.