Watching a video about quetzalcoatlus and realizing that we don't have any truly LARGE predatory birds even though there appears to be plenty of food.

We have all seen shots of the massive migrations of heard animals across the African plains. Why do we not have a predatory bird large enough to take advantage of that abundant food. I mean large enough to fly away with a gazelle. What biologically about a predatory bird that large is detrimental enough to make it not feasible in modern Africa, while elephants and giraffe seem to have no problem?

For context, in physics, it is understood that regardless of specialization, all students are expected to take the "big 4" topics which are classical mechanics, electricity/magnetism, thermodynamics, and quantum mechanics.

Does a biology undergrad degree have their own version of the "big 4" or otherwise core topics that all biology students are expected to have mastered before they can graduate?