The Square Cube Law.
"This principle states that, as a shape grows in size, its volume grows faster than its area."
So a thirty-foot-tall human should weigh
substantially much more, in proportion, than a regular-sized human, would need incredibly strong bones in order to stand, and would need a heart pumping at very high pressures to actually have its blood get anywhere. It would also need larger feet, since the human-proportioned feet would drive themselves into the turf because of their smaller surface area creating very high ground pressure.
But if somehow this
Fifty-Foot Whatever (trick URL!) is somehow much
less massive than it should be, it could work... (though this would have to include its fluids and whatnot).
However, as noted, the Titans still hit the ground, walls, buildings, livestock, and people with the force of something that weighs as much as they should if they were that tall, instead of hitting like they were made of cotton candy and vapors. This is... quite contradictory.