You've got that backward. If google fires off a request when you open the email, the companies can track whether you opened the email.

If google fires off a request when they get the email, the company has no way to track if you opened their email.

If google fires off requests to all emails they receive to @gmail.com then the marketers won't even know if it's active @gmail.com account, they'll just know that *@gmail.com is received, which isn't information, it's an easy assumption.

Actually, if they do it on delivery, it ruins the signal (since there is no longer any distinguishing factor between active accounts and inactive accounts), so the latter protects privacy more effectively.