Nobody does that. The biggest problem the author didn't really relate to is that most Python packages come with no signature what so ever. So, what are you going to do about those packages if you wanted to verify the rest? -- just verify those that have signatures, and leave those without signatures alone? -- That just sounds ridiculous...
By and large, PyPI is the same story we had with HTML: incompetence that breeds more incompetence. The same way how Web browsers encouraged crappy Web developers to generate ever more broken HTML by "helpfully" fixing the broken parts, PyPI by not enforcing the good practices created a community of people who now don't even understand what the problem is.
Fighting this is hopeless, because security is by far not the biggest reason why people download and use Python packages. So, even if security sucks, the users, who, ultimately should've been the ones responsible for their security won't do squat. Instead, maybe, at best, there'll be some broken / partial solution from PyPI.