This is widely viewed as the proper way to do a startup, and if you work on a different schedule, you are likely to be told you are “doing it wrong” by employers, investors, and peers.  This is peer pressure deployed as a weapon. 

Partially it is simple hazing: the experience of shared suffering in the group context is believed by many to form stronger bonds.  (And, as with classical hazing, “I put in my dues, now you have to put in yours or I will look pretty freaking stupid now won’t I” causes the system to perpetuate itself.)

In addition to the hazing rationale, some portion of this attitude is just pure exploitation.

Astute.