Simple equation: The more they can access your iPhone + that precise data is fed into mega data centers parsing out your historical behavior patterns and the grouped historical behavior patterns = more $$$$$. The incredible incentive is clear as a bell: Get our hands on that data and rake in the dough. But it is even worse than it seems on the surface. iPhone users are the coveted demographic, they have substantially more discretionary spending power than Android users.
Facebook, Google, Spotify and now Amazon even Microsoft plus a myriad of medium to small tech companies that rely heavily on this user specific data aren't actually ranked on their products or their user happiness. They are ranked on revenue, specifically ranked on ever growing revenue (to be fair, Apple is too). Down to brass tacks that's it, period. Facebook could jump 5 million users in a quarter, Wall Street is thrilled, but then they state their revenue is sloping down. They are now a flailing company. Facebook was a trillion dollar company less than a year ago. It is a 380 billion dollar company now. That is A LOT of wealth erased. Not all of that but a fair portion is because Apple comes along and says "sorry, we are making it easy for users to not let your app reach into all parts of an iPhone to record everywhere you go, who you call, what sites you visit, what words you are using etc etc etc (even Peloton called out Apple's ATT for hurting revenue).
The incentive is clear and two things were sure to happen. One is numerous companies would be calling up their political and media contacts to go after Apple. You don't erase a trillion in wealth without a war happening. Second, any slight gray area that exists with ATT will be exploited by Facebook. To be fair ti Facebook you can be sure that many companies are thinking of doing it or even are doing it. Interestingly the only company almost certainly not doing it will be Google. ATT cut off some data to them but due to the popularity of Google Maps, Google Search, Google is a winner with ATT.