Let's do some math.

Surface area of a torus is 4 x 3.14 x 3.14 x R x r.

Using 15" for R and .75 for r:

4 x 3.14 x 3.14 x 15 x .75 = 444 square inches.

At 100 psi, that's a total of 44,400 pounds of pressure. High pressures in large tires mean literally thousands of pounds of force trying to tear your rim apart.

And as others have already said, harder tires do not roll better.