Because it’s a different domain. I expect google app links to go to something that looks like google (or goo.gl, etc), not something totally different that I am not knowledgeable is part of google.
This change isn't about what Google apps do. Indeed I wouldn't be at all surprised if Google Photos sharing continues to use photos.app.goo.gl following this change as it does now.
This is about turning down a service that formerly allowed arbitrary third parties unaffiliated with Google to make goo.gl links going to whichever destinations they wanted, and allows those third parties to track certain usage data like bit.ly also allows.
Google is simply removing that third party offering. I think that aligns very well with what you'd prefer, if I'm understanding you correctly.
It wouldn't surprise me if reducing user confusion is even part of their internal rationale for the turndown, though I don't actually know. I haven't been privy to such details since I left Google roughly 3 years ago.
