I'm not stating that they should be. I first want to make sure this is not just a question of the "Real ID". I can think of a couple of reasons that would throw a wrench in the works:
- passengers on no-fly lists or criminals
- anyone who is underage -- do we let 10 year-olds fly alone? how do you assess age without ID? what if the child gets lost while traveling, and you can't even determine whether the child boarded their flight or not? (if you attach ID to the ticket, then that just seems like ID with extra steps? I could be missing something)
- baggage claim: if there is no link between ticket and person, what's to stop me from claiming anyone's luggage as my own?
I'm not firmly attached to any of these objections, actually -- and perhaps they're not even issues, because I'm missing something fundamental about the assumption. I admit my personal bias is that "taking a plane = passport" even when traveling domestically (I'm not a US citizen), so I have not thoroughly considered the possibility that "taking a plane = taking a bus".
> anyone who is underage -- do we let 10 year-olds fly alone? how do you assess age without ID?
Most children don't carry ID. And most ID for children doesn't even have a photograph. For travel that doesn't require a passport, you just have to deal with asking the child or their travel companions.
> baggage claim: if there is no link between ticket and person, what's to stop me from claiming anyone's luggage as my own?
I can't recall being challenged as to my identity for any luggage I've picked up from the carousel. This was as true in 1997, when it was the focus of a major motion picture [1], as it is today. Regardless, having possession of the matching luggage receipt for a tag should really be sufficient to pick up a bag.
> I admit my personal bias is that "taking a plane = passport" even when traveling domestically (I'm not a US citizen)
That's a reasonable personal bias, but presuming you're subject to mandatory alien registration and you're eighteen or older, you have a legal requirement to register and carry proof of registration. Citizens are not generally required to be registered with the federal government, or to carry proof of registration or other identity documents; although enough things require a social security number that the vast majority of citizens are registered and enumerated by SSA, and enumeration at birth is the default, but a very small number of citizens manage to be born without a birth certificate being issued.
[1] https://en.wikipedia.org/wiki/8_Heads_in_a_Duffel_Bag