Why Have Birds Never Gotten as Big as T. Rex?

I read that the oxygen content of the atmosphere was significantly higher then:

"The atmosphere of the Earth 80 million years ago was discovered to have 50% more oxygen than modern air. Brenner and Landis found that for all gas samples taken from amber 80 million years old the oxygen content ranged between 25% to 35% and averaged about 30% oxygen".

From: https://www.npl.washington.edu/av/altvw27.html

Sounds like being supercharged with oxygen could support larger mass on land.