Wasn't Outer Heaven in a jungle? Makes sense for that since your base could get bombed by airstrikes and have your silos destroyed. No MBT could handle the trees and swamps of that biome and by the time you get jets there the things already gone so you have to rely on grunts to fight that thing, and the railgun launching the warhead means your enemies won't even be able to react with counter ICBM tech. So it makes sense for MG1, the rest of the games seem to just shove Metal Gears in because it's in the title.