About 75% of the time I don't get asked for ID at all when flying within Schengen Europe. I understand technically there isn't any border crossing, but they have absolutely no idea who is actually on the plane. Wild.
That's because you have the right to travel freely between Schengen countries.
Asking for an ID or passport when embarking/disembarking would be similar to having an ID check at a border, and would therefore be a border control, i.e. no longer free travel.
Of course Schengen countries have the right to temporarily re-enable border checks.
I imagine it's the same as a police officer asking for your name on the street. They are allowed to ask, and you are required to provide it. There's no restriction of freedom happening. I think it's logically prudent to know who is on a flight.
Asking for an ID or passport when embarking/disembarking would be similar to having an ID check at a border, and would therefore be a border control, i.e. no longer free travel.
Of course Schengen countries have the right to temporarily re-enable border checks.