Ground power? Low profile is the name of the game for ground power. No matter how good your armor, it is always cheaper to create a weapon that can destroy it. The rule for ground power is 'smaller is better' as long as it can fulfill it's job… Tanks are incredibly vulnerable without infantry to protect them, and the 'bottom' of the combat effectiveness size pyramid is a human being with a weapon… still the best combat creation for it's scale in existence.

Air power? It's radar cross section is HUGE… instant kill via a far less expensive aircraft.
Ocean power? It doesn't float, and if it submerges the noise from a humanoid shape like that would attract torpedoes like flies.

Don't get me wrong, mechs are VERY cool, but until a defensive development that is insanely science fiction… such as force fields, such a device is simply impractical for anything resembling combat. And even WITH 'improbable' science advances, much more compact forms than a mech would still utterly dominate such a pointlessly complex device.

There are millions of possible NON combat roles, however, that a walker could excel at… construction, Minesweeping, demolitions, disposal, medical, hot zone fortification, and hazardous environment to name just a few…

But mounting a pair of machine guns instead of those arms (or anywhere but perhaps on a top automated pintle mount for dealing with nuisance infantry) would literally wreck any possible usefulness it possesses compared to vehicles that would cost a fraction of it's expense.