Because the mass of something is proportional to its volume, while its strength is proportional to the cross-sectional area, a giant robot -- a giant anything -- will eventually become so big that it can't support its own weight. There would be similar problems with the power supply, plumbing, and heat dissipation. Basically, the humanoid form we all know and love only really works if it's about yea big.
Where "yea" is about as big as you are. I'll assume you're sucking in your stomach.
Sure, if you get one of these robots thanks to a genie or captive wizard, many of these problems would be magicked away. But not all of them. All your robot's weight would be resting on two relatively tiny feet, which would send you plummeting into the ground with every step. Imagine wading through a swampy field after a hard rain -- the kind of thing you lose a shoe in. That's what even the most basic stroll in your giant robot would be like. Walking in a built-up area, on concrete and asphalt, might help a bit, but that's going to wreck the roads. And raise the stakes. Because then, even the smallest mistake will lead to a car getting crushed or just the worst day ever at a petting zoo.