It's because Washington, DC is the nation's capital and where our legislative bodies, heads of state, and our various departments (ministries) are based out of. There are also many military bases in close proximity and the military headquarters (Pentagon) is across the river in Virginia.
I think people are misinterpreting my comment... there _are_ non-defense/government jobs in the area but they are dwarfed by the number of such jobs because of the pervasive nature of the aforementioned entities. When I lived in the area more than 90% of LinkedIn recruiter messages that I got were about these jobs. This ceased being the case after I moved even when I was still in the defense contracting industry.
I think people are misinterpreting my comment... there _are_ non-defense/government jobs in the area but they are dwarfed by the number of such jobs because of the pervasive nature of the aforementioned entities. When I lived in the area more than 90% of LinkedIn recruiter messages that I got were about these jobs. This ceased being the case after I moved even when I was still in the defense contracting industry.