I don't think it's the physics per se, but rather the convenience of it. The films haven't really established how durable a lightsaber is. I think the closest we've seen is when Darth Maul got his cut in half by another lightsaber. How does a lightsaber survive whatever battle it went through unscathed (presumably something that involved a star ship exploding), then float through the vastness of space for who knows how long, then get pulled into the atmosphere of a planet?
I mean, you can say it's made of outer space substances that are really strong or whatever, but by that logic couldn't they have done the same thing with the droids in ANH? Like, if they had jumped out of the airlock towards Tatooine rather than used an escape pod, couldn't that be sold to the audience just as easily? They're robots made of space materials, they can survive that.