Because, according to our laws, if you don’t want someone to traverse your land or hunt on it you have to say so. Posted means that they’ve informed you that you can’t go there. Laws vary but it means no trespassing and that you cannot claim ignorance as a defense, because you have been informed.
https://www.quora.com/Why-do-some-private-property-signs-have-the-word-POSTED-in-huge-letters