Not all of them, in fact CO2 are very small in numbers compared to dry chem.
Yes, absolutely, thermal runaway, cells ignite others cells next to them.
They actually are, come to a large city and look for yourself, in NYC probably 70% are underground, or inside a building, that pretty much definition of enclosed. Even open multilevel garages, not much different, fire spreads sideways. smoke fills the space, and make it impossible to see, or work without SCBA. not to mention heat compromises structural integrity of a building.
You may think that, but real world fact show that concern is as real as it gets.