Please explain why there is no higher latitude than 90 degrees N and 90 degrees S. .

The answer to your question involves simple Euclidean
Geometry. The Earth is round (more or less) therefore if viewed on a flat surface, its
form would be a circle. A circle is comprised of 360 degrees. The Equator, which divides
the world North and South, is at the mid point of that circle, so this divides the
circle into two halves of 180 degrees each. In measuring latitude, one begins at the
Poles, which are at zero degrees and 180 degrees, similar to the Equator, only they are
drawn at right angles to the Equator itself. A line from the North Pole to the South
Pole, will, with the Equator, divide the world into four equal parts, each of which will
be 90 degrees on the arc created. So, if one measures from the North Pole at zero
degrees to the Equator, one measures 90 degrees. Again, when one measures from the South
Pole at 180 degrees to the equator, one again measures 90 degrees. If we treat the
Equator as zero degrees latitude, then the distance (or angle) to each pole is 90
degrees. North or South depends on the direction in which one is moving. Hope this
helps.