But that "opportunity" is there because it's a secondary or tertiary choice for employers. Considering that 70%+ people are commuters, a) there are more desirable locations no matter where you're coming from, and b) there's no direct way to get there, other than Route 1, which is going to be a disaster.
No major highways or arteries, Metro goes through Arlington first, etc.
If you live in DC, you have to leave the city. If you live to the west, you'd prefer Dulles, Tysons, or Arlington. If you're to the south, it's Alexandria. To the north, Bethesda. To the west... probably Baltimore, or you have to drive all the way through DC.
Dulles is no better of a commute for most people in the DMV, especially those that want to live within the DC/Arlington area.
They could have gone to Dulles and had a blank slate to build on, but they don't think their employees and prospective employees want to live there. It's at least an hour from downtown during rush hour. It's an exurb, and Amazon does not see that as the future.
However, there are major organizational and political hurdles that would have to be tackled to put any new infrastructure to effective use, so I wouldn't hold my breath. But maybe Amazon's choice of location could spur things in the right direction.
https://www.washingtonian.com/2018/11/13/did-amazon-just-ren...
"National Landing was coined in the joint Northern Virginia pitch to Amazon at the start of its search process last year."
https://www.bloomberg.com/news/articles/2018-11-13/amazon-pi...