There's probably lots of reasons. I always got a kick out of it when I was up before dawn. I'd think, "Everyone else is sleeping, and I'm awake and alive!" That's a nice feeling.
But I don't think that's the biggest reason.
I've been on the nocturnal sleep schedule plenty. Sometimes I'm in Asia, and I have calls back to the USA during USA-daytime. That means I'm awake at the night, and sleeping through the morning into early afternoon.
It's fine to wake up at 2PM, as long as you start working immediately.
But if you don't start working immediately, and you halfway screw-off with your time for three hours, then you're in trouble. Very good chance the whole day is sunk. It's 5PM and you haven't done anything... yeah, you've only been up three hours, but that... just... feels off. Very hard to get going full steam.
But if you wake up at 4:30AM, 5AM, whatever - and you have a leisurely morning, slow breakfast, screw off and waste time for a few hours - then it's only 8AM. Still good to go.
Hell, if you waste the first seven hours of the day, then it's only noon. Very cool.
Whereas if you get up at 2PM, waste seven hours, then it's 9PM and you haven't done anything... yeah, good luck with that.
Lots of nice things about getting up early. But I think that's an underrated one - if you burn a few hours, it's still very early in the day. Psychologically that seems to really mean something.