There must be a historical reason for this, and that is the question I submit to you, dear readers: Why are Shabbat leftovers so often put into sandwich or quart bags when a reusable container seems so much more reasonable?
When I see this, both my inner hippie and my inner organizer hurt. Besides the obvious waste and harm to our environment, it is nearly impossible to stack bagged leftovers in an already-full fridge. Let's not even speculate on the potential for leaks and spills both in the fridge and while filling the bag.
I definitely understand the heavy use of disposable eating ware on Shabbat (though that also triggers my inner hippie), and I'm guilty of that on a regular basis. But bags? Plastic reusable containers are better for the Earth, your fridge, and for stuffing that fridge with the most food possible. What's the downside?
I actually cringe when this happens. But when I ask why, the answer is always the same: "Hm. Never thought about it. We've just always done it this way."