Joe Biden refers to Kamala as 'President Harris'

This is not the first time that President Joe Biden referred to Kamala Harris as president or president-elect. He has done this before and it was even fact-checked as TRUE by Snopes. Why does Joe Biden do this? Is he losing his mind? Is he making a joke? Does he just forget sometimes? We're not really sure and don't have an answer for it, but in my opinion, it's somewhat embarrassing and not really funny anymore.