Why are honorary degrees awarded to celebrities?
The "degree" part of "honorary degree" is largely irrelevant.  Honorary degrees are simply honors bestowed by a university, and they are considered degrees just because degrees are what universities hand out: a degree is the one distinction not available elsewhere in society.  Honorary degrees do not necessarily have anything to do with study or recognize any scholarly accomplishments, even in the loosest sense.  (This is true both historically and today.  There has never been any strong connection between honorary degrees and scholarship.)  Scholars can be awarded honorary degrees, but so can anyone else.

In particular, politicians, donors, and celebrities are sometimes honored by universities for entirely non-academic reasons.  For example, in the United States a university typically awards an honorary degree to the primary speaker at its graduation ceremony, who is chosen for fame and speaking skills.  The speaker might be a distinguished scholar or might have no scholarly credentials whatsoever, but an honorary degree will be awarded either way.

There's nothing wrong with this, and there's no reason why universities should restrict their honors to scholars.  The only strange part is calling the honor a degree, but in practice people who care about such things know it's not a real degree.  Honorary degree recipients occasionally take the degree part too seriously (by bragging about it or insisting on being called "doctor"), but that just serves to make them look foolish.