Take your pick: Gundam, Pacific Rim, Godzilla, RoboCop, MechWarrior, whatever. When it comes to giant robots in pop culture, they're almost always in the hands of the military, sent to fight against some equally giant threat.
In film and television, it makes for a great, over-the-top action sequence. In reality, if troops were given a giant robot to battle, they wouldn't be focused on the awesomeness our childhood selves imagined, but rather on all the annoying chores associated with a real, giant, fighting robot.
6. Maintenance would be a pain in the ass
Think about how troops handle Motor Pool Mondays today. Kicking a tire, turning it on, and sitting in the A/C doesn't count as an actual maintenance check of a vehicle.
Imagine if you had to PMCS a vehicle the size of a building. Nobody would check every inch of that robot to make sure it works.
5. The rules of engagement on deploying one
Whenever anything is deployed into a combat scenario, risk assessment forms are done out the ass just to make sure that using a certain piece of equipment is worth the risk it poses to its surroundings. This is why the big guns of the Apache's Hellfire missiles aren't tossed around like candy.
Now, take that risk and multiply it by every step the robot takes, every laser that it shoots, and every time it punches a giant monster into a building.
4. You probably wouldn't be the pilot
If you consider how many people share the dream of being a pilot in the real world versus how many pilots there actually are, you'll understand your chances are slim.
In fiction, it always seems like a young and spunky kid is given the reins on a multi-billion dollar fighting robot and everything works out. If the military hardly trusts its troops with something that costs a few thousand dollars, good luck getting behind the reins of a 10-figure fighting machine.
3. If you WERE the pilot, you'd be uncomfortable as hell
Go ahead and ask a tanker, pilot, or literally anyone who spends their career operating heavy military vehicles if they were physically comfortable in their vehicle. Unless they're a fighter pilot with seats designed to withstand the G-Force, they'll laugh at you for asking such a ridiculous question.
Every last dime would go into giving it the ability to dispense more firepower and take more hits. Uncle Sam doesn't care if your legs get a little bit sleepy.
2. All of the safety classes...
Still enthused on the giant robot idea? Well, consider that the military would likely make a million and one different classes on the importance of proper robot safety. You'll zone out and start hating the robot the well before you're through with half of the forty required robot safety courses.
1. It'd just suck in battle
And then cold, hard reality sinks in. Giant fighting robots just aren't that effective in battle.
If they're designed to walk on two legs, it could trip easily because of how top heavy it is. If it had hands, the controls to match the precise movements of a human hand would be mindblowing. If it was as massive as a building, it would be such an easy target. If it was piloted by a human, the human better hope the metal casing is sophisticated enough to him or her.
If you manage to do mental gymnastics to justify a giant robot in the face of all these issues, congratulations! You're basically describing a modern day tank — and that already exists.