In Death of a Salesman, why do Biff and Happy leave Willy at the restaurant?

Act 2, scene 5 of the play Death of a Salesman,
by Arthur Miller, takes place in a restaurant where Biff, Happy, and Willy
meet for dinner. The dinner was meant to be an all-male family meeting in which they
would discuss Biff's success in his business endeavours with Oliver. Keep in mind that
Willy still sees Biff as his great hope. However, Biff is slowly beginning to realize
that his life has been a lie created by Willy and perpetuated by
himself.

The realization comes when Biff's plans for
business finally meet with reality: Oliver makes him realize that he is not his business
partner, but just another worker in the company. This is the final step for him
to realize his fake sense of self importance. As a result, Biff tells
Happy:

I
realized what a ridiculous lie my whole life has
been!

This incident alone
already sets the mood for the rest of the night by foreshadowing that there will be more
self-realizations about to occur. Indeed, they do occur. First, Happy tells a call girl
in the restaurant a lie about Biff saying that he is a quarterback for the New York
Giants. This lie is almost compulsive and typical of the Lomans because it is based on
the idea that the Lomans still hold true that Biff is some sort of big shot
footballer.

That sours Biff's night even more, because it
leads him to realize that his entire life has been a lie, propelled primarily by his
father and his own false believes. Now, here is Happy doing the same
thing.

Moreover, when Willy appears in the restaurant, the
lie is meant to be perpetuated when Happy tells Biff not to tell Willy about the fiasco
with Oliver. Hence, now Biff is more sure of the type of lie his family lives in. Yet,
Biff is not willing to take it anymore and decides to confront his father with the
reality of the situation. Happy does not agree with what Biff is doing and insists on
pretending that everything is OK.

Yet, when the scene comes
to a blowout, Willy leaves the scene presumably to the bathroom and about to enter a
flashback. Meanwhile, Biff and Happy argue, sending Biff off in anger and Happy
following with the two escorts he met at the
restaurant.

This being said the argument with Willy, Biff,
and Happy at the restaurant is caused by Biff's decision to face the reality of his life
and making his father realize his own reality as well. We see how neither Willy nor
Happy are willing to see things for what they are. As a result, Willy takes off and is
left alone when Biff leaves angrily and Happy follows.

This
scene is the first attempt of any of the Lomans to accept that their lives have been a
lie. It is hard to accept something of this nature, and the result is clear: Some rather
run away from reality than confront it.