You just aren't going far enough into the purchase process to see them.

If you just select the initial "View Seats" option on your flights, they will not show as open. You have to click "Continue" after selecting flights, enter your passenger info (should automatically fill in when logged in), and then click "Continue" again to get to the "Review & Purchase" page. At that point, you can click the "Select Seat" option for your flights and it will allow to you view and select open exit row seats. Note that on some aircraft, some exit rows are also Comfort+ and you will not have access to those as a Silver until the 24 hour mark.