[ratww]

Because it's not as effective if the goal is to catch spam. Spammers are already wise to the meaning of + and will strip it automatically when selling data in bulk. Plus, some services block creating accounts with the + or with their name in the address.

[pixl97]

Block any email to the address missing the +

[m-p-3]

Then you end up with spammers simply putting gibberish after the plus sign.