The people operating these imprinters are sales clerks and waitstaff, not graphologists or experts in detecting altered physical credit cards. The sophistication of fraudsters has also advanced, and as a result, a system that might have been good enough in a pinch 20+ years ago isn't necessarily good enough today.
That said, in my view there's no excuse to not leverage the physical chip present on effectively all credit and debit cards these days, which is technically capable of making limited autonomous spending decisions even with both the issuer and terminal offline in scenarios like this. It probably won't happen without regulatory pressure, though.
Unfortunately, too many cards, and all mobile wallets, don’t support offline authorization for that to be viable.
In fact, even verifying the signature is no longer required in at least the US.
Signature verification also only solves cardholder authentication, not card authorization (i.e. figuring out if the card is funded, still valid etc.)