>>16780019
>Maybe the ancient world had more cliffs and such to protect flying animals nesting sites too who knows.
>Perhaps having more islands or shallow seas may help flying creatures specifically

I didn't think to mention it in my post here: >>16779649 but what the other Anon said is also another excellent reason as to why the sea is so important in producing large birds: privacy.
Shallow seas and tall coastlines produce precarious or otherwise difficult to reach places where large seabirds can nest in peace and comfort. Think about Albatrosses, Boobies, Frigate Birds, Penguins, and the unfortunately extinct Auk: all rather large birds that depend on nesting sites in massive private colonies on rocky outcrops where food is readily available in the reefs nearby, or at the very least their chicks are safe enough that they can leave them alone for long periods of time.