Typical armchair expert. Have you ever trained? You wouldn't last even 3 minutes shoot boxing if you haven't.
"Just get fitter!" May as well be "just get smarter!"
There's a heap of variables at work...your genetic ability, your build, your style...ect.
People naturally reference Conor when it comes to cardio, but he carries a fair bit of muscle, is extremely explosive and is a pressure fighter. We've seen his measured approach in Diaz II, and he still gassed. When you're constantly relying on that fast-twitch, explosive counter striking, naturally you're going to burn more energy than someone who's willing to sit outside and bide their time.
Genetics you can't change, and all the elevation masks, breathing exercises and altitude won't change anything if you have a poor disposition for endurance and you're a pressure fighter.