Why can't you reuse BBs in an Airsoft gun?

Because they get dirty and might even have tiny cracks that could cause them to really screw up the internals of your airsoft gun. If you just have a $5 plinker from Walmart, it's not that big a deal, but if you have an actual airsoft gun that cost you hundreds of dollars, it's not worth ruining it with a 2 cent BB.