I am guessing: unless you blast them in a blender, the beaten mix isn't really uniform and you may end up extracting more egg white or more yolk than intended when you remove some of this lumpy content.

At that point you may as well start worrying about the variance of the yolk to egg white ratio in different eggs.

^ yea pretty much this

I'm not 100% sure but whenever I've tried to reduce an egg without splitting first, it always ends up with a super wrong amount of yolk

I could also just be bad at it, idk