*They're notes worth taking. And worth considering, although they might not know it, is that Samus started with a basic power suit, and has added upgrades piecemeal as she finds them. In the time when the Chozo actually needed armies, every last Warrior had such armor, and the upgrades in which they had learned to be proficient. Multiply Samus by ten, a hundred, a thousand, and you begin to imagine what the Chozo could bring to a fight--and, perhaps, why they have not needed to for so long that the modern galaxy considers them pacifists.*

It's complicated. The simple explanation is that they needed to do things outside conventional space, and their living corporeal forms weren't up to the job, so they had to leave those behind. I wasn't ready to go, so I stayed with the living.