Is there a particular reason for this, or it was just not created ?
PKCS#7 specifies the Cryptographic Message Syntax CMS. A small part of this is a padding for blockciphers, generalizing the padding from PKCS#5 to other block sizes.
Many questions mentioning PKCS#7 are about the block-cipher padding. I don't think it's useful to use the same tag for questions about the padding and CMS.
Introducing a tag for CMS seems useful, but I'd tend to name it cms not PKCS#7 to avoid confusion with the padding. I found 4 questions related to CMS.
Introducing a tag for PKCS#7 blockcipher padding is a bit dubious. I'd either keep it at the current padding, or introduce a separate blockcipher-padding, to distinguish it from the rather different RSA paddings.