Because relative to the quantity of email Gmail has to process, the images returned from tracking links are likely a drop in the bucket, and requesting them all provides their users with even better privacy protection than the "Show Images" toggle did.

If you want to be cynical, you can note that Google will still know which emails you opened and which you did not. Does the current Gmail TOS restrict them from selling that information to advertisers, or (more likely) using it to target ads? Probably not!

I suppose if they're clever, they'll figure out when a sender is serving a million copies of the same image to slightly altered URLs in the same email template, and forgo the requests, but either way, the sender loses the analytics.