Because USATF doesn't want to tacitly encourage or deal with hordes of course-cutters turning in OTQ chip times. USATF doesn't have the resources to vet hundreds of entry submissions, holding to gun times is the simplest route to ensuring integrity.
I ran Houston from the A corral, I'm way slower than this woman and submitted a half time I ran at MP. She didn't want in the OT badly enough if she didn't prepare better to take her best shot and line up factors to her advantage when she easily could.