It's your standard superweapon problem. Why put all the money and resources into a single massive weapon that can be destroyed when you could instead make a lot of smaller, more practical stuff for the same price? We don't make battleship sized tanks, and not just because of the physics problems they'd have trying to move on land. Hell, we don't make giant battleships for the sea any more either, and that's the one place where there isn't a physics problem.

What advantage is there to using a giant robot? What does it do that's more effective than existing vehicles, drones, weapon platforms etc? Sure, it can walk over terrain that might be hard for some ground vehicles, but we already have shit that can fly. It could have multiple weapons to choose between if it uses them by hand, but we are still paying for those and could instead mount them on cheaper platforms and use them all at once. Other than style points, the only thing I can think of is the utility that comes from having giant hands to pick stuff up. Being a heavily armed combat crane is probably not worth the investment.