Apple can't strip identity from the existing trackers because there's not a separate and distinct part of the tracker that encodes the user identity. It's integral to the tracker itself, which makes this an all-or-nothing proposition.
<img src="https://example.com/cd726f02-d2f4-4c0e-a717-e69044180c59.gif" height="1" width="1">
The image filename is a UUID. The UUID is of course unique for each email sent, but the Web server is configured to serve the same image for any given UUID (after recording the UUID as an "open" into a database).
There isn't a way for an email client to be certain that the image is or isn't a tracking pixel.