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.