I mean the same reason Apple uses your phone to scan for nearby AirTags.

This isn't a feature that is actually costing them sales but a lack of DRM/etc affects what apps will be in their store.

It certainly costs them some sales, but not enough for them to care about.

Why would Apple ever prioritize their customer's interests over their own? They've never once suggested that they would, and their customers prefer a hierarchical relationship. Apple is a company that whitelists which functions of a general purpose computer that their customers will be allowed to use.

That makes some people feel really secure, like the company is a loving parent, although companies don't love. They decide what is profitable and what is not.

That's a weird hill to die on.

Service providers need to ban people sometimes. This includes people who are savvy enough to know how to delete and reinstall an app to clear its settings. Never permanently banning anyone simply isn't a thing that's happening.

If Apple didn't provide DeviceCheck, or something similar to it, service providers would use some other means of deterring abuse. There's a couple directions they can go in, but they're all generally worse for users (e.g. using invasive tracking, requiring users to pay for service, etc). DeviceCheck is about the least invasive way I can imagine this being implemented.

Why do mail providers work against your interests by blocking outgoing spam? Because in aggregate it's beneficial to users if external parties can trust the more.

That's different because you block spammers' messages on your servers, without the cooperation of the spammers' computers.