Since Link is a character in the Zelda universe, he reincarnates just like everyone else. It's just how life, death, and rebirth function in the Zelda universe. Give any dead person so much time, and they will eventually be reborn into a similar body with a similar name.

Is every Link a reincarnation?

Each link embodies the Hero's Spirit which is unmalleable by all accounts so it remains pure and all that stuff. Link doesn't have real reincarnation cuz he doesn't know about his past selves except as legends or myths or similar.

Why does Link and Ganon reincarnate?

Ganondorf has only ever been reincarnated twice, in FSA and way before BotW's oldest backstory, and we know very little about those incarnations before they became Ganon apart from his FSA incarnation being a Gerudo male. The reason Ganondorf has only reincanted twice is because he keeps cheating death.