That's really the core issue. Developer-signed packages (npm's current attack model is "Eve doing a man-in-the-middle attack between npm and you," which is not exactly the most common threat here) and a transparent key registry should be minimal kit for any package manager, even though all, or at least practically all, the ecosystems are bereft of that. Hardening API surfaces with additional MFA isn't enough; you have to divorce "API authentication" from "cryptographic authentication" so that compromising one doesn't affect the other.
How are users supposed to build and maintain a trust store?
In a hypothetical scenario where npm supports signed packages, let's say the user is in the middle of installing the latest signed left-pad. Suddenly, npm prints a warning that says the identity used to sign the package is not in the user's local database of trusted identities.
What exactly is the user supposed to do in response to this warning?
Imagine a hobbyist developer with a ~ $0 budget trying to publish their first package. How many thousands of km/miles are you expecting them to travel so they can get enough vouches for their package to be useful for even a single person?
Now imagine you're another developer who needs to install a specific NPM package published by someone overseas who has zero vouches by anyone in your web of trust. What exactly are you going to do?
In reality, forcing package publishers to sign packages would achieve absolutely nothing. 99.99 % of package consumers would not even bother to even begin building a web of trust, and just blindly trust any signature.
The remaining 0.01 % who actually try are either going to fail to gain any meaningful access to a WoT, or they're going to learn that most identities of package publishers are completely unreachable via any WoT whatsoever.
That's really the core issue. Developer-signed packages (npm's current attack model is "Eve doing a man-in-the-middle attack between npm and you," which is not exactly the most common threat here) and a transparent key registry should be minimal kit for any package manager, even though all, or at least practically all, the ecosystems are bereft of that. Hardening API surfaces with additional MFA isn't enough; you have to divorce "API authentication" from "cryptographic authentication" so that compromising one doesn't affect the other.