Why do we celebrate "two time, three time etc champ" thing?

[famicommander]

Everybody except a select few loses, especially at the top level.

Showing you can rebuild yourself into a world champion again after taking a loss is the next most impressive thing to remaining undefeated.

And as others have pointed out, you don't have to lose to be a multi-time champion to begin with. Changing weight classes, retiring and unretiring, etc.