Yes they could, they would, and you know it. They'd be serving you malware, they'd look even worse, they'd be for scams, they'd sell all the data they harvest from you to 1000 other companies, etc.

I don't think they can. People expect third-party apps to sell them stuff, but not their OEM.

Apple used to differentiate themselves by not using their OEM apps for ad space. Now they're copying the exact same "curated" promotions style that made Windows packin apps intolerable.