Same reason you're not supposed to extinguish an oil fire in you frying pan by splashing water on it.

it doesn't foam

I though this was "common knowledge" but here goes.

There are different class fire extinguishers depending on what is on fire.

https://vanguard-fire.com/what-are-the-5-different-classes-o...

You cant use water on any sort of "gas" fire because the gas (airplane fuel, etc) will simply float on top of the water and remain on fire. Throwing water on it can actually help it spread as well.

In recent fire truck vs airplane crash on the airport they used water and it seem to worked well.

We're talking about open air fire treated with carcinogenic foam in large quantities where 1m+ large flakes land in somebody's private garden.

Water alone:

https://youtu.be/3ZKBBtwNaRI

Water with foaming agent:

https://youtu.be/WVOmm4Fw6hc