We don’t do it because of safety. The rule book states we must be at a complete stop before releasing the doors for one thing. But because drivers (and guards where involved) can make mistakes - stop shorts, overshoots, and wrong side releases), most TOCs encourage drivers to pause before opening to make sure all is well. I believe SE request that their drivers physically go to the other side of the cab to release doors on the offside. Far better to avoid incidents in the first place, not least for passengers who may step into thin air, but also keeps drivers on track which prevents shortages. Once we have more comprehensive automation we can have faster door releases.