Are athletes/celebrities the only sports/entertainment companies who implement the concept of brand extension? Why or why not?

No, they're not.

Explanation:

Because any main product who has a good reputation can use it to launch another secondary product, for example, imagine a company like Google [who it's not a celebrity or an athlete] lauch a small company like L'Gooface who creates smart makeup [makeup with technology], and in the promo they use Google reputation to drag the attention to the product and therefore people will trust in the product and small company.