With the recent addition of Velocity on Vatsim compared to what it was years ago, it changed a lot. In +5 years we got flaps animations, all lights triggerable, gear, sounds and lower latency !
This mostly answers it.
Took 5 years to sync over the internet those basic animations, which are not even "streamed" over the network, the only thing that gets transmitted is in fact the standard variables that holds the status of these system and/or the events that caused it, like keypresses. The actual animation is played locally, no need to transmit it.
Instead, GSX animation are completely custom, and they also need to synchronize with each other.
Something like the guy that loads luggage, for example, requires a precise synch between the animation that picks up a bag at a specific frame, then it drops it at another specific frame, and this across several different objects working together, the baggage truck, followed by a series of baggage carts holding the bags that must also advance at the right moment, the guy that picks them, and the belt loader animation. They must all be synched to the frame, otherwise the animation would look wrong/choppy, and I'm afraid it might not work over the internet, where latency and speed are unpredictable.
