They may have done that. It may have made it clearer to them that they were transferring over a lot of trust to someone else, and they might have taken it more seriously and used a different approach. It can be helpful to have more friction for something like this.
That's true, but I have the (perhaps wrong) impression that most people would take handing over control of a signing key more seriously than a repository on GitHub or a package on npm. It very explicitly involves trust / security so it's harder to ignore what should be obvious.
I get the impression that the original developer didn't deeply consider any security implications of handing over control. Maybe they would have just handed over a signing key too. I don't get the impression they acted in bad faith and deliberately put people at risk though.
The same thing can happen due to a bad / reused password. They're trusting the password choice of all the dependency authors too. Of course, a developer could also had their computer compromised, etc. Having a hundred dependencies from unknown / unfunded developers is a problem.