Or are airplane passengers invariably saints, so that this never happens?
Even the most opportunist thief probably doesn't want someone elses old clothes and half empty tins of hair product.
Even worse if it's a return flight - that means the clothes are dirty!
Same reason why theft from tents on campsites is not (in my experience) so prevalent. Tents are vulnerable to burglary but most of the potential burglars are other campers and are therefore also vulnerable.
The laissez faire baggage claim is refreshingly delightful in contrast to the hell that every other aspect of air travel has become though.
It is also not uncommon in places where petty theft is high for you to have to show your baggage receipt matches the baggage tag before you can take it from the baggage handling area.
The exception is baggage claims for international flights which are usually in a secure area due to the need to pass through customs.
My favorite airport was Long Beach; it's been ~9 years, but last time I flew in the baggage claim conveyor belt was literally outside at the curb. (And this is why that place rocked. The expansion has likely since ruined all the little conveniences of LGB.)
Once upon a time some baggage areas had security which would spot-check claim tickets before you could leave the baggage area. That was mostly pre-9/11, though, when checking in your bags could be easier than leaving with them. Some major airports have [now] uselessly annoying barriers to baggage claim areas for that purpose.
Not sure how it could be any other way. After all you can pack weapons in checked bags, so once you are at checked baggage you are already beyond the security perimeter.
Use one way doors like pretty much any airport in Europe. Once you go to baggage you can’t go back to the main airport. But you are still not in a public area.
Are they enforced with guards blocking people from coming in the exits?
I suspect it's risk based. If they have reason to suspect there's ongoing theft, then they add additional layers of security.
Outside of that... theft would be extremely risky. There's a good chance the owner is already at the carousel somewhere watching their bag come out, and there's cameras everywhere.
For domestic flights in the US, yes, generally anybody can just waltz up and take a bag if they wished.