Because once you run into somebody who decides that your subjective judgement represents illegal discrimination, you’ll wish that a third party checked the boxes you.
And it's not evidence of illegal discrimination if you eventually decide to hire somebody who didn't have the "required" level of skills or experience but submitted their resume anyway and you happened to give them an interview anyway but found them much more impressive in person?
If anything, it would seem to me that the more you put down on paper, the more legally exposed you are if you don't follow it to the letter. Of course, this doesn't absolve companies whose hiring managers do stupid things in interviews that are almost guaranteed to get their company sued; but that's a concern to be trained against regardless of how discerning your textual job posting is.
