there is a reason why such giant creatures don't exist anymore.

Yes, it's because of an asteroid.

Large dinosaurs were significantly larger than large mammals for basically the same reasons that birds can fly much longer and higher than bats: hollow bones and significantly more efficient lungs. Flying birds can also get much larger than bats, despite both breathing the same atmosphere.

Dinosaur lungs worked the same way bird lungs do. Their lungs are rigid, and there's separate sets of air sacs that work like bellows to pump the air through the lungs in a single direction. Much like fish gills, there's cross-current gas exchange so they cab extract most of the oxygen from the air.

Oxygen levels weren't really any higher. Dinosaurs just had some adaptations that let them both get bigger than other groups of animals and be better at flying.

The better question is why large birds haven't re-evolved. That's probably just due to the success of mammals. 10 ft tall "terror birds" evolved in the Americas, but went extinct within the last two million years due to competition from mammals. Birds could get larger than elephants, but first an elephant-sized bird needs to outcompete elephants.