There's nothing actually stating that he's stuck in beast form on the DT. We do see him in his beast form exclusively from ALttP to Zelda 1, but that doesn't mean that he's incapable of switching between forms when we've seen him do it before. Hell, in The Wind Waker, he's capable of changing back and forth (and it's implied that he was in beast form during the era of the Great Flood), but doesn't never turns into a beast.
Saying that Ganondorf loses his human form on the DT is no more than an assumption. All we know is that the entire DT has the same Ganon from OoT(yes, even BotW/BotW 2; CaC outright confirms that the Ganondorf that became Calamity Ganon was born in the same era as the Gerudo Sage of Spirit, Nabooru), and that he's insane from Oracles onward.
It's also interesting to note that while the Gerudo of BotW have pointed ears, CaC mentions that they once had round ears long ago(which they had during the era of OoT), and that Ganon's corpse had round ears when his human form never had them in the DT.
