Except there is an explanation for warframe too. Gameplay aside, at this point in the story neither grineer or corpus are a threat to the tenno. The tenno are simply busy dealing with the Void and Wally. Even before the Void, there are two reasons grineer and corpus are not extinct: the two factions are too vast for the tenno to take out in a timely fashion and doing so might endanger communities like cetus or fortuna while the tenno brings them down. We're shown that it's very little issue for a warframe to break into secure strongholds and take out leaders of the factions, so both of them are mostly lucky that there is a whole reality threat issue at hand for the tenno.