Why did these actors makes so few movies in the 80s?

Well, Pacino was in Scarface (1983)...one of his most iconic roles. Ditto for Dustin Hoffman and Rainman (1988). Both worked in the 80s, just not quite as prolific as in other decades.

Marlon Brando was on a downswing since the 1970s and wasn't in anything that was a commercial success after Apocolypse Now.

Sidney Poiter's starpower faded, perhaps partially due to the rise of other marquee black actors in the 1980s (e.g. Eddie Murphy, Denzel Washington, Morgan Freeman, Carl Weathers).

wasnt Marlon Brando notoriously tough to work with

Probably a combination of factors.

They weren't being offered enough scripts worthy of their reputation, directors figured they could get good performances out of actors who weren't such a pain in the ass, the actors had made their pile and wanted to enjoy life a bit, and they were asking for too much money because of their big names.