Because there will eventually be a 2-2 option alongside 2-1 Tarkus. The idea is to have several planets for most if not all legs of your journey... eventually.
So it may ends like one chain of plot planets with mostly lust-resistant enemies and other dmg-resistant? Or some along the road some changes will be made to not divide it thais way? As now if new 1-2 and then 2-2 turns out to be with many Amara-like enemies it would be again splitting people that will rush throu Mhen'ga+Tarkus or 2-1+2-2 to get to ant planet :/