Every walled garden wants complete control over their "harvest". Apple tried to make an Ad business, but later pivoted back to hardware. Once the iPhone sales slumps, there is no saying they won't attempt this again.
Don't forget Apple's ad revenue soared when they made users be able to opt out of tracking.
And, oh, right! Their own tracking setting was (and probably still is) hidden in the App store of all places because of course they don't have to play by their own rules.