Why can't NFL players hold on to the ball before they score touchdowns?

"In the Bengals' win over the Titans, Jordan Battle fumbled the chance for a touchdown by celebrating too early, resulting in a costly touchback for his team."

"Jonathan Taylor faced a similar fate, dropping the ball before crossing the end zone during what was originally ruled a touchdown, leading to another touchback and missed opportunity for the Colts."

Read at www.theguardian.com