They arent when you consider the limitless nature of the galaxy the Imperium inhabits. We are talking a million worlds spread over an incomprehensible distance from each other. It wouldn't matter if there was a googol of people per world, it still wouldn't even be a fraction of how much space is in the galaxy.

Terra isn't tithing troops; it is a festering hive world that requires a countless number of people to keep running, plus billions of servitor, and religious pilgrims.

Not to mention most worlds will typically have mega fauna to deal with in addition to incredibly hostile environments to deal with. Not to mention most, if not all of the worlds wouldn't be able to sustain the amount of human life that Terra can.