Why? Because if they catch fire or explode, the resulting fire can develop so quickly that you may not have time to get out of the house.
If you charge it while you’re away and it blows up or catches fire, and you’re in a freestanding home, at least only the home would be damaged.
A plane that had just taken off from San Diego, for Newark, had to return a few weeks ago when a battery pack or a battery in a laptop caught fire. Four cabin crew were taken to the hospital as a precaution.
They had manage to stop the fire by putting the object into a thermal containment bag, but I could imagine that that sounds much easier than it is in reality.
Watching this video made me realize that it’s far worse than I thought. Hmm.