Why doesn't Hap accept Biff's invitation to go west to start a farm?
Like Biff, Happy is Willy's son. And like Willy, he is a businessman who thinks his self-worth is measured by the mark he makes in the office and the money he brings home. Here it is in Happy's own words:
I gotta show some of those pompous, self-important executives over there that Hap Loman can make the grade. I want to walk into the store the way he [his boss] walks in. Then I’ll go with you, Biff. We’ll be together yet, I swear.
Of course Happy has no intention whatsoever if going out west with his brother Biff, no matter how much he swears. Again, like his father, Happy is full of hot air.