Yes, we're all simply unfit sloths, standing up is physically demanding and that's why standups must be short. :)
Seriously though, my impression is that standing up encourages brevity in other ways.
Standing implies that the meeting must be short because there apparently isn't even time to sit down before you start. It really does save time when you do not have to find a suitable and available conference room and let everyone pick a chair.
Standing also makes subtle signals of loss of focus very visible. You easily pick up on the small things, like when people start shifting their weight, look at their phone, turn around to look at other things, etc. Things that aren't as visible when sitting down.
There is also a implied invitation to walk away at any moment. You're already standing, walking away requires little effort, at least physically.
Seriously though, my impression is that standing up encourages brevity in other ways.
Standing implies that the meeting must be short because there apparently isn't even time to sit down before you start. It really does save time when you do not have to find a suitable and available conference room and let everyone pick a chair.
Standing also makes subtle signals of loss of focus very visible. You easily pick up on the small things, like when people start shifting their weight, look at their phone, turn around to look at other things, etc. Things that aren't as visible when sitting down.
There is also a implied invitation to walk away at any moment. You're already standing, walking away requires little effort, at least physically.