Facebook, Instagram and Whatsapp are all fine apps on the surface but deep down they collect so much information that Apple is very bothered about it. Facebook, you see is in the 'people' business. (They want to collect as much information about you, me, my friends, my family members, your family members & all its users basically they want to collect as much information as they can) and make sure they use that information against their own users.
FB wants to know everything about you if you boil all their T&C's in simple terms; your likes, your dislikes, your hobbies, your interests, the websites you visit, movies you like to see, where you are etc.; they want to know as much about you so that they can build you a very lucrative profile about you and use it to target you with those creepy good very specific ads and pour money in their own pockets out of it and also sell your information to the third-party brokers and company. It's actually scary not only how much they know but also how much they want to collect. Apple and Facebook are sword companies at this time. They do not like each other at all.
Specifically, it all has to do with Apple's firm stands on privacy. Apple has taken a more conscious effort over the past few years to really double up on the privacy, safety and security of the iPhone, iPad and every single device in the Apple ecosystem as a whole. They want to give the users more control over the use of their information and really just make sure that the photos you take on your iPhone are safe and secure until you choose to share them. Or as you browse the internet on your iPad, you are not being tracked along the way and you are not being served ads based on what you do in an app or what websites you look for. Apple is very privacy conscious and FB is not willing to do that all and they are kinds of doing it in the opposite.
Apple changed this privacy game with the launch of its 'App tracking transparency' feature in its iOS 14 software update. This feature gives users the choice of whether they really want to be tracked across the apps and the websites. This feature changes the whole power dynamics between the users and the companies and hence gives you super control. This one single press of a button in the iOS has the ability to set down millions and billions of dollars to the FB industry.
Apple is trying to restrict FB as much it can. FB still works as a normal app. They can collect as much info about you in their own app but they can't track you around the web. FB wants all this to go away but Apple is making every part of this data collection process, every link in the chain as transparent as they can.