Out of sheer curiosity, why are you playing the game at 720p on a 1080p monitor?
There are a lot of people who stream at 720p and don't want to deal with the quality loss that downscaling incurs. Streaming at native game resolution means providing a 1:1 video feed, ensuring that in-game text stays sharp, and highest fidelity is retained. So they'll play at 720p, even if they have a 1080p monitor, and suffer the not-that-great image quality on their end to ensure they're streaming the best possible quality.
Also when streaming retro-games, 720p is the 3x multiplier for 240p consoles, keeping the pixels nice and sharp.
Streaming at 1080p with the higher bitrate requirements that carries can dramatically reduce your potential viewerbase. Twitch (for example) released data a while back showing that most viewers will be able to watch with minimal buffering events at the 2000-2200kbps mark. Even going to 2700kbps started a serious increase in buffering. Going to the previous ingest recommended maximum of 3500 caused a STEEP falloff in potential viewerbase.
People who are buffering aren't going to stick around. They aren't going to complain about the buffering. They'll just leave to find another channel to watch. The only ones who will stay are the ones who don't buffer... so it's VERY easy to get into an echo-chamber and assume nothing is wrong.
There's a lot of people who just crank bitrate to the max their connection can handle, up to the recommended ingest maximum, and end up shooting themselves in the foot as far as channel growth is concerned, because they got lost chasing numbers.
There's very good reason that 720p 30fps 2000kbps is still the 'sweet spot' for channel growth, even with the recently increased recommended-maximum 6mbps ingest rate on the table.