Everything that Rock's been doing over the past few years has been in service of his presidential aspirations. He's teased it in multiple interviews before, and his character in Young Rock was very specifically an attempt to pull a Zelensky (TV president running for the real thing). His curated PR, branding, and efforts to create this mythologized version of his past are all building up to it.

He doesn't have the political experience, but Americans will vote in celebrities if they have perceived business/corporate experience. Trying to become a bigwig in Universal/Warner Bros/football were part of that plan, but it hasn't been successful and (+ the Maui thing and some low-revenue films) has hurt his standing in Hollywood and his overall brand. WWE is just another potential stepping stone to get where he wants to go, as well as a way to rehab his image a bit like he did 10 years ago.

Rock doesn't watch wrestling/isn’t familiar with how Cody got to the position he's in, and genuinely didn't expect this anger. I don't think he knows why WWE's previous output allowed something like AEW to take root to the level it did, why the recent HHH-led changes have led to an increase in fan engagement in WWE, or Cody's significance to both of those things. For him, coming in and big-leaguing Cody is like taking things over from the Miz - who would fucking care? But that doesn't seem to be what's happening, and whatever crowd reactions he gets week-to-week, it's a real risk that at least the WrestleMania crowd will heckle the shit out of him. As much as getting booed will bruise his ego, the advertisements that political opponents can put together with footage of tens of thousands of people booing him and chanting 'Fuck off Dwayne'/'Die Rocky Die'/'Bullshit'/etc. - even if it's just one crowd that does it - are going to hurt him way more. WWE/working with Roman aren't the goal - if he thinks it's going to hurt his eventual candidacy, he'll pivot (and say he did it for the fans)