What value does that provide? As the installer of something, you almost never personally know the developer. You don't really trust them. At best, you trust the operating system vendor to sufficient vet contributors to a blessed app store. Whoever published package A is actually a maintainer of Arch Linux. Whoever published app B went through whatever the heck hoops Apple makes you go through. If malware gets through, some sort of process failed that can potentially be mediated.
If you're downloading a package from PyPI or RubyGems or crates.io or whatever, a web repository that does no vetting and allow anyone to publish anything, what assurance is this giving? Great, some package was legitimately published by a person who also published a public key. Who are they exactly? A pseudonym on Github with a cartoon avatar? Does that make them trustworthy? If they publish malware, what process can be changed to prevent that from happening again? As far as I can tell, nothing.
If you change the keystore provider to sigstore, what does that give you? Fulcio just requires that you control an e-mail address to issue you a signing key. They're not vetting you in any way or requiring you to disclose a real-world identity that can be pursued if you do something bad. It's a step up in a limited scope of use cases in which packages are published by corporate entities that control an e-mail domain and ideally use their own private artifact registry. It does nothing for public repositories in which anyone is allowed to publish anything.
Fundamentally, if a public repository allows anyone to publish anything, does no vetting and requires no real identity disclosure, what is the basis of trust? If you're going to say something like "well I'm looking for .whl files but only from Microsoft," then the answer is for Microsoft to host its own repository that you can download from, not for Microsoft to publish packages to PyPI.
There are examples of making this sort of simpler for the consumer to get everything from a single place. Docker Hub, for instance. You can choose to only ever pull official library images and verify them against sigstore, but that works because Docker is itself a well-funded corporate entity that restricts who can publish official library images by vetting and verifying real identities.