While we do see several corpses throughout the laboratories, the complex is mostly deserted. If most of the staff were evacuated during the experiment, why didn't they make themselves known after things went wrong? It could be that the same circumstances that brought the dinosaurs into the future sent those people into the past where they now have to fight for survival in a Fate Worse Than Death.
The dinosaurs ate them.
Why there are so many carnivores. The Third Energy's exchange targeted the very Late Cretaceous, after the meteor impact had kicked up the dust cloud and started a nuclear winter. Plants died first, then plant-eaters, with scavenging carnivores briefly thriving before they eventually ran out of prey. It was just before this final extinction point that some of them were transported into the modern century. Also, the Therizinos are a previously undiscovered species, not genuine Therizinosaurs. - Blue Chameleon
Alternatively, the opening cutscene shows that there is an entire jungle out there surrounding the complex. It is entirely possible that whatever herbivorous dinosaurs arrived are out there, but the predators were drawn to the facility because it was loaded with weak, slow prey items that were not prepared for their arrival. Namely; humans. Why bother with a prey item that can run away from you or fight back in a vast jungle, when you basically have a lunchbox full of weak, frightened, powerless creatures?
After watching a Lets Play/Walkthrough on YouTube, and replaying the game. I asked my brother why would the T-rex smash through the window to eat two people? And it hit me just as he said it. The T-Rex is a fish out of water and hungry because of it. This also explains why it acts as a Super-Persistent Predator throughout the game. It is in a time completely different to its own, the raptors and Therazinos which would be food sources are hiding inside and there are no herbivores for it to hunt. It smashes through the window because it is a brightly light box on what is otherwise and grey lump of stone and it sees movement in there. So it goes for it. This also explains why it eats Cooper (The guy with the radio) in the opening, it sees a beam of light and hears movement as it investigates so it starts running. Of course humans only provide so much. And with Regina it because a Road Runner vs. Coyote situation in wanting THAT one human. It attacks you the second time because it hears the antenna being activated. It attacks the heliport because it sees the lights and hears the sounds. Going after the big helicopter first because it would supplement him, but since it didnít taste good it throws it to the ground and goes after the next food source, Regina. Later it follows the research workers. But it canít eat them because the elevator is too small for it too move and ends up stepping on them. And once it gets out into the underground it sees you and knocks itself out due to space. In all three endings it tears down the facility tracking you as you are the last thing it saw before passing out. Plus there's that pride thing. Your smell wakes it up in Gailís ending and it follows you to the hoverport in Rickís ending. Gailís trail of blood also makes it go to the hoverport first in the final ending. Ė Doc Yoshi, Wishing he hadnítwritten this so late.
While an odd possibility, there is also a chance that Regina is starting to become her own niche within the newly developed ecosystem. The T-rex may be a territorial creature, and perhaps feels that as Regina slaughters her way through the base, she is attempting to establish dominance. The T-rex, being the supreme predator, can not risk another supreme predator (Regina) invading its territory and potentially threatening not only to starve it, but push it out of its established hunting grounds. Of course, the animal has no idea that Regina is just fighting to stay alive, but as far as it is concerned, Regina is a rival to be eliminated at the first available opportunity. It isn't all that odd of a concept. In any ecosystem where two predators of equal capability interact, the result is often the predators attempting to drive off or kill each other whenever the chance comes along.