I don't think anyone has said the middle seat holder is "entitled" to a switch. The consensus seems to be that you are hoping for an empty middle (don't we all), and when it doesn't work you offer to switch because your preference is to sit together not with someone between. The middle seat holder likely recognizes that you played the game and it didn't work, so hardly feels like you're owed much.

If you're resentful of not getting thanks in this scenario, then don't bother offering. I have done this multiple times with my spouse - if we can get an aisle and window we do, and then let it ride.