Throwing a bit of anecdotal evidence out: throughout high school I had to get up at 6:45 every weekday for four years, and constantly felt tired throughout the day, but it cleared up at night and I'd be energized at 1am. I had to take naps after school, but even if I didn't I'd still be unable to sleep at 11pm. Something about everyone else around me being asleep and not distracting me allowed me to focus on things better.

Now I have a job with very flexible hours so I go to sleep at 2am and wake up at 10am like clockwork. I feel and perform much better all around. I'm convinced that if I had had a sleep schedule that worked for me in my teens I'd have been able to achieve much more than I did.

I think your claim (and probably mine) is too broad. It sounds like most people have a fair degree of flexibility, and I'm skeptical about our depth of knowledge in this area. For example, this:

When they wake early, for example, night owls are still producing melatonin. “Then you disrupt it and push the body to be in the daytime mode. That can have lots of negative physiological consequences,” Wulff says, like a different sensitivity to insulin and glucose – which can cause weight gain.

I couldn't find the study where that came from, but I wonder how long it lasted. Even now, if I revert back to staying up late, it can take a few days of getting up early before it feels amazing.

All of which is to say that I suspect that genetics plays a role in our natural preferences, but our bodies seem remarkably adaptable to different lifestyles.

Also, I think it was probably unfair of me to say "it's not that hard", because it took me years to get to the point where getting up at 4am wasn't that big of a deal. In retrospect, the key is almost embarrassingly obvious: go to bed early (9p). I don't take melatonin or anything, but I've always had a really easy time falling asleep, so others might find that helpful in getting over the initial transition of early bedtimes.