My 79 yr old like to stay in bathrobe all day. Asked him why he doesn't get dressed. He says he isn't going anywhere, gets angry if I ask

Well, he isn't going anywhere, perhaps, but he IS somewhere and that is your home. I agree with RealyReal that you have a right to dictate how people living in your own premises will behavior for the most part. If he is ill for a day of course, is is understandable, and he may be slow off the mark getting out of his robe. I am up early but often lounge about in my robe for several hours. RealyReal is correct that for the most part we feel better when we are dressed.
If he has his own area where he stays for some time, a room and living room and bath, then I wuld think that gives him more freedom for his choice of attire.
You ask why he does this. Could be depression. Could be he is tired and giving up. Could be he is tired and wishes he were gone and be more at peace. Sit with him and explore the subject.

Your house, your rules.

Getting up and dressed just makes you feel better. That's why he should do it.

Getting angry because you ask is unacceptable and should be addressed in a way that makes him understand that it will not be tolerated. He doesn't need to get ugly and if he does then you should tell him that he can move and then he can do whatever he wants, but as long as he lives in your house he will be respectful and that means getting dressed and dealing with oral hygiene and treating everyone in the home with kindness and respect.