Surely, there must be other ways to control you partner from standing.
Yes, There are several ways to apply osae waza standing or even "half-kneeling". Ways which also use toris hands or at least one hand. Or ways of doing osae waza using toris legs.
The way we teach osae waze there often is a part of technique where hands are changing or shifting when sitting down. (sankyo, nikyo e.g.) During this part the legs support waza. This support can be elaboratet as a part of technique of it's own. This is the way we come to learn to apply oase waza with the legs, letting the hands completely free.
When practicing buki waza (i.e. tanto dori, jo dori, tachi dori) we apply osae waza standing. One hand holds the weapon of uke (and maybe can use it ...), the other one applies the lock.
The point of aikido osae waza must be something else. What is built in your body, what does the body learn from performing osae?
Doing osae waza sitting seems to teach centeredness a lot. And a clear connection to uke. No tolerances, no possiblity to adjust by moving ...
Things like that. I think ...