QR code has very limited data capacity. (According to Wikipedia) one single QR code can only carry <3KB of binary data.
It's not rare for a PSBT to hit this 3KB limit, especially for ones which spend non-SegWit UTXOs.
SegWit features signing of input values - on the other hand, signing non-SegWit inputs on offline/hardware wallet still requires full data of previous transactions to validate input values. Otherwise, it could be a security issue: a malicious party/malware could tamper the inputs stealthily to trick the user into paying an unexpected high amount.