Reviving old batteries?

First off, Just a riding mower battery. I wouldn't consider this for my car battery. Anyway... I've watched a few YouTube videos where the person dumps the original acid fill, then uses Epson salt and baking soda solutions to clean and flush the inside of the battery. Then they'll fill it with something different, Cider vinegar in one example.
So my question is, Why not put the original acid back into it? To me that seems simpler/easier than trying to properly dispose of the acid. Educate me