I think that's a consequence of spherical symmetry. I'm not sure I have the entire logic worked out, but because it's isotropic, then the distributions need to be uniform I think.
It's peculiar to the 2-sphere ("ordinary" spheres).
I imagine there is a trend where, in higher dimensions, coordinates have a greater tendency to be near zero?
In a 0-sphere, x is either -1 or +1. In a 1-sphere, each of x, y, is (informally speaking) more likely to be near +/- 1 than near 0. A 2-sphere gives us uniform distribution for each coordinate. So I suppose that the coordinates of a 3-sphere are more likely to be near 0 than near +/- 1, and this tendency is more pronounced, the higher the dimension gets. (?)
n-spheres are funny things. Intuition about them is often misleading. (See, for example, the comments expressing skepticism about my original uniform-distribution observation.)
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EDIT. Ooo, some interesting questions here. What is the limiting behavior of the distribution of x on the n-sphere, as n -> +infinity? I imagine the graph looks like a narrower & narrower spike at x = 0.
Now, suppose we scale the graph of the distribution horizontally by some appropriate function of n. That is, let f_n be the probability distribution of x on the n-sphere. Is there some function g:Z -> R so that the function x -> f_n(x / g(n)) has a limit as n -> +infinity? Does it approach (wild guess) a normal distribution? If so, is this fact (even wilder guess) a special case of some kind of central-limit-theorem-ish statement that holds for geometrical objects?
Thanks for the paper link.
In fact the simplex seems to be the worst case for convex objects, in terms of concentration of distribution near the center. And the best case should be the sphere. Which plays out nicely since the simplex seems to be the most "concavey" convex shape of a given 'diameter' is the sphere is the most "convexey" convex shape of a given 'diameter', no?