I thought there were Israeli private services/ contractors providing APT as a service to, for example, Saudi Arabia or other despotic regimes.

I think that was in the news back in the sochi Olympics. The value of cyber capabilities is only going up with time

The siloing may be due to multiple contractors. I imagine these exploit vendors are protective of their arsenal of attacks.

Because as has been said many times, the three letter agencies aren't exempt from the curse of government employee mediocrity.