I believe that your question is why Linda, Happy and Biff continued to have dreams and hopes of a better life, financial success, and a good life whereas Willy did not.
In a way, Willy was a broken man. He knew that he had reached his limits and limitations. He sticks to the past and lives in past episodes because he understands that there is no future in sight for him but death. What would happen to his family, especially Biff?
Linda is an enabler who lives in oblivion and denial.She knows her husband is not well, but is completelyairheaded about his career, hence, she is still "allowed" within her denial, to believe in something.
Happy and Biff are literally "happy go lucky" and would never settle down for anything if not for quick money. It is precisely that what motivates them to "dream" if that's how it canbe called.
It is basically that each family member lives in their own sense of what reality is.