A unique identifier which is 100% stable over multiple sessions for once. Which makes the rest of the data so much more useful.

There are plenty of ways to uniquely enough identify a user on the web.

Why is that possible in native apps but not websites?

In the end, this is because the browser works for you, and the app works for the company.

You mean the browser developed by a adTech company, the browser developed by the company who gets $18B a year from an adTech company to be the default search engine or the browser created by a company who only survives because of the adTech company?

My guess? They don't only get a unique identifier, but they get to know and link you to other more useful data using your Apple/Android device ID and so on, that they can use to track you across multiple user accounts. Something that browser fingerprinting can't guarantee 100% (but getting very accurate these days)

Apple specifically limits apps ability to get a unique identifier that can be used across apps. And the percentage of people who use multiple accounts is not even worth optimizing for.