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Possible reasons why are to the average player the sentry post seems pretty useless. Other than using it to see invisible but theres no need to upgrade it if thats all you need it for. Sure you can call down paratroopers but they take a hell long of time to actually get there and they usually get killed straight away. Sure you can mass them but that takes time and resources you need for stuff like your wall, towers and stuff that don't run away when nothings left to kill (if they come to your base at all).