Why must black celebrities speak out?

The first episode of OJ: Made in America chronicled O.J. Simpson's rise to celebrity and contrasted his non-controversial persona to that of other star black athletes of the era. Although there were indications of a self-obsession associated with sociopathy, which might explain his later criminal conduct, I had no complaints with Simpson's decisions at the time.  

If he had an obligation to speak out for social change, then Joe Namath had no more and no less of an obligation.

Likewise, if Michael Jordan had, or has, an obligation to speak out for social change, Larry Bird has no more or no less an obligation.

How is it NOT racist to hold black athletes to a higher standard?

The obligation DOESN'T EXIST. Not for anyone of prominence, of whatever race or background. Doing what you love to do, and not hurting anybody else while you do it, means you're helping society.

It's unreasonable to demand anything more from anyone.

In any case, if a celebrity "speaks out" for anything other than more liberty, what they really are demanding is "social change" for the worse.

It's better if they stay silent.