The same reason why you aren't allowed to put lithium powered stuff in your hold luggage on a plane.
Indeed, thermal runaway is not advisable when you're 40,000 feet up.
What happens if it goes off in the cabin?
It's not like you can chuck it out the window is it?
Only solution, I suspect, is to place the offending item into an oven and shut the door. In reality this will involve a degree of serious injury, but it might just save the aircraft.
At worst, at least a lithium fire in the cabin would be more easily detected and therefore give time and options, rather than the Valuejet incident which went from 'why doesn't that work?' to total death via 'why has the floor melted? ' in all of two minutes.
Also, there's significantly less chance of a lithium battery fire in the cabin... Usually they are from a dropped phone/tablet getting down the side of a seat and the seat being moved, crushing the phone/battery... If this happens to you, DO NOT MOVE YOUR SEAT.
In the hold, there is significantly higher chances of moisture shorting the battery because the holds aren't routinely heated, so condensation is a real probability. Ever wondered why your suitcase is soaking wet when you collect it and it's not raining? It's because that really annoying family of seven you spent all holiday avoiding and the whole flight wishing you were sat in business, on another flight, spent the afternoon in the pool and put their wet towels in their suitcases.
Fully charge a laptop battery or power bank, and then spray it with a light mist of water, and see what happens.
But back to your original question, I will let you know when I do my SEP course at the end of the month.