Because you already bought the phone, they hate you and just want your money. I’m sorry, but it’s the truth. Source: I’m a victim of this as well.

The fact you can do it for proprietary apps only shows that.

1. Apple is fully capable of doing so for any/all apps. Actively, right now, it wouldn’t even be hard.
2. Does not care about the convenience of its customer base, let alone the complaints and frankly decades of user input and suggestions.
3. Is greedy (like always), they don’t give a shit about giving you a good product, just one that’s ’good enough’ to take your money, but they have to ration commonplace features so they have something new for next year (that has been a feature elsewhere for 5)

Watch, in a year or two they’ll try selling the new model and tag it as “Now with the ability to set sounds for ALL your apps! Amazing! WOW! Innovation!”…… it’s classic Apple, they think they’re the best because their head is so far up their own ass that all they see around them is shit.

Let us fucking customize the UI and give us the ability to set ALL notification sounds as we see fit. Cut the BS and stop limiting features that are standard on basically every other device/software available. It’s 2023, Jesus Christ.