Testing whether a account number is valid before transmitting to the server is a problem that was identified and solved over 60 years ago with the Mod10 algorithm. Why aren't they using it?

Right now, the only "does this credit card number look right" validation they do in JS is to look for a known prefix digit, and the length of the number. Otherwise the send it to the server for processing.

This is silly. In 1954, the Mod 10 (AKA Luhn Algorithm) was designed specifically to catch people mistyping in account numbers and all credit cards use it. It's a check digit, where the last digit of your credit card number is based on the all the other numbers. It catches transposed digits, and mistyped digits, which are super common when asking a human to type in 15+ digits.