Willy's reminiscences Why is Biff upset with Willy's mumbling reminiscences?

Biff is bothered by the
way that Willy refuses to live in reality. When Biff was in high school, Willy fantasized about
Biff's great football future and made excuses for Biff. Even then, Willy was engaged in his
dreams more than in the reality of the world around him. Biff can see that Willy does not
recognize reality much of the time and he feels this is a choice Willy is making. 

Effectively, this refusal to engage in reality means that Biff and Linda are ignored by
Willy. They are invisible to him in their reality.