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.