Since many homes have only one, and must be used by children as well as adults, the 'standard' height is low enough to allow most folks to have their feet on the floor as much as possible, and to bring the knees 'up' a bit, giving the body the most efficient angle for for easy elimination.. (I had an entire book on the design of bathrooms once)
IF one needs other heights, they can be ordered from specialty plumbing suppliers.
IF one has really, really special needs, it is possible to make the toilet adjustable in height. I know someone who designed one, bought necessary parts (sleeves, hydraulics...etc) and installed it to make use by handicapped persons easier. It works fine! has about 6 inches adjustable range, I believe.