As explained in more detail on the FAQ page I linked above, we cannot influence whether a third-party payment processor's site would auto-redirect the buyer to any page after completing checkout -- e.g., PayPal and Google Checkout only present a link at the end of their checkout process, which the buyer must choose to click through manually in order to proceed to the thank-you page.
This is also why we send a thank-you email with a link to the thank-you page for download products, so those buyers can still reach their download page even if they did not choose to proceed past the end of checkout, or if they could not do so because their payment was still being processed by that time.
If you are not selling downloads, then the buyer has no reason to ever click through to a thank-you page, so tracking conversions via thank-you page visits would be unreliable if you're using PayPal or Google Checkout.