Why does the USA uses companies like Boeing and Lockheed to make there military aircraft and other military eqipment?

Why would they do that. These companies sell to hundreds of other countries as well. I don't understand why they buy these aircrafts from these companies instead of making there own aircraft. Hire high tech builders to build it and make it there self. It just seems to me to be a safety issue because a another country lik china could easily buy or undermine the company in some way to obtain these high tech fighter jets. This means the leaders of these companies have the blueprint High tech jets of mass destruction.