I can think of one big case: Many NFC tags are used as QR codes. Encrypting this doesn't really get you anything, since it's public information that's being bro

I can think of one big case: Many NFC tags are used as QR codes. Encrypting this doesn't really get you anything, since it's public information that's being broadcast freely to anyone anyway. On the flip side, adding the extra hardware to a tag to support encryption has a non-trivial cost. (NFC tags are limited by both storage space and power.) If you're doing mass production of tags, that cost adds up very quickly.