Because it works

(AIUI "I agree" gets people to give explicit consent with good success and the subscribe modals are quite effective too)

There was a piece of caselaw about a company having an "agree" but a million tick-boxes to disagree. A lot of sites added a "disagree" button then.

That doesn't take away from the fact "I agree" etc can be displayed prominently and often is. "I agree" isn't outlawed