Airlines that are strict on size and weight of a carry on don't have kiosks so you can't easily slip it by them.
Also the bulk of QRs traffic is international, with an extremely diverse set of citizenships and access requirements such as visas, visa on arrival, ETAs, reciprocity fees, number of blank pages required, number of contiguous blank pages required etc. etc.
Even on AA. Kiosks do passport checks, but any time I've had to get an actual visa or produce some sort of documentation, I've had to go to an actual staff member.