Why would anyone agree to this?

Because the opt-out is hidden under the third pane behind the per party 1 by 1 drop-downs of the opt-outs for "legitimate interests", in a list of 120 "vendors" that require individual sub-pane drop downs and a radio button all having an "agree to selected" button that will immediately negate all your previous selections and just blast you to the content as if you "agreed to all".