As someone who develops apps for confidential conversations, making it harder for people to screenshot the confidential stuff is a feature the sending party wants, that is why they send in your app as opposed to others. It doesn’t make things impossible, just hard enough that 95% of people won’t bother to take a copy.
Same for example with disappearing audio messages on whatsapp
What I don’t like is the app being informed that I took a screenshot. The OS can hide things in screenshots without this.
In a world where people have multiple old phones lying around it isn't that hard to come up with this workaround.
If you send it, it is no longer yours to control.
If it is my phone, it should be mine to control. Too often it really isn't my phone...
Another way to look at it is the OS makes certain guarantees to the developer around security. Giving control of this to the user would erode that guarantee from the OS to the developer. The result of that is that some developers would simply never display some information (e.g. due to their own contracts or reasonable concerns about fraud/abuse/etc.).
Very similar to the video pipelines in modern devices. Prior to video pipelines which the OS could attest could not be hijacked by the user, many content providers simply would not allow e.g. Netflix to release their content on certain platforms. That the OS does provide such an attestation option for developers allows uses that otherwise would not exist.