My cousins lived in fire prone mountains in northern California and they had a swimming pool.
They had a gas electric generator and a large submersible water pump attached to sprinkler system that could drench the entire house.
If there was a fire threat they would fire up the generator and evacuate. The system could run by its self for 12 hours or until it ran out of gas.
The system cost less than a thousand bucks.
These fire suppression systems are not that expensive as you pointed out. Especially IF you have a 20-50000 gallon reservoir(swimming pool) in the back yard.
You would think that a person who owns a $5-25 million dollar house could afford to do that.