How does someone die 8 hours later of hypovolemia in the the emergency room!?

i am 18 years old and my mother ped away when i was 13. dec. 17th 2003, her and her boyfriend robbed a gas station in lufkin, Texas. during the get away chase, they crashed head on with another car in a residential neighborhood. my mom was on the pengers side and wasnt wearing her seatbelt so she was thrown from the car. then the cop that was chasing them ran her over. on her death certificate it says that the main cause of death was hypovolemia. then a pelvic fracture with hemorrhage. and the last two causes were a splenic laceration and respiratory failure. she died 8 hours after the accident and she was air lifted to the hospital. now how does someone die 8 hours later in the emergency room? couldnt they have given her blood? isnt that why we donate our blood to these blood banks? in those 8 hours couldnt someone have stopped the bleeding? checked for internal bleeding? something!? ive watched shows where people have gotten shot in the head and still lived! i just need someone to clear somethings up for me... any e.r. doctors that can help me understand this???