Here are the reasons: 1) rarely care what client the user views the email in; 2) any request to the server is enough information to identify the user, email, and open/received; 3) a tracking pixel is an antiquated form of tracking; 4) the list provided doesn’t include some big names (FB for one).
The only effective way to stop the tracking, that I’m aware of, is is to cache the result of the request. Even that doesn’t stop newer techniques. Instead what you need to do is to cache the request and then apply a model to determine if a similar request leads to a similar result for other users and serve that from the cache. To the best of my knowledge no one has built that yet, and could probably be circumvented.
The alternative expensive solution is to download EVERY image a user receives and store it indefinitely. That way the trackers aren't any more useful than "was the email received".
In short, I'd love a better solution to " load all images " vs " load no images " but that's where I ended up talking to our frontend engineer at Fastmail. Obviously he had thought about the problem a lot more then me (I do operations not any frontend) and there's no easy solution.