Google says there are about 50,000 killer whales right now. They are apex predators and similar in mass. Of course sea creature versus land animal, but also I believe we are at a low point for killer whale population.
1. Water covers ~70% of the Earth's surface. Whales migrate of course but can exist from the poles to the equator;
2. Of the 30% land, for pretty much any species, some portion of that is uninhabitable. Dinosaurs being reptiles likely require hot and wet environments. As predators, T. Rex would be limited by where their food could live too.
The dinosaur era was largely one of a super-continent (Pangaea). A good portion of that was likely too cold or too dry to support such a large predator.
They lived in what is now Western North America, but was then an island continent called Laramidia [1] so yes, they were geographically quite limited.