Hollowing out the river, building a high tech security complex right beside it, and landing two dozen quinjets on the surface would be difficult enough to hide.
Having three massive doorways hiding something obviously meant to emerge from them, even more so.
The three carriers were either built in the heart of Washington (and if you look at the photo of them deploying, they're in sight of the entire city) and were put through all their flight tests by flying over the capitol. Or they were built and tested elsewhere and flown into the city at night, with four massive repulsor engines glaring over everyone.
Since they don't think the reflection panels would hide the new engines, or that noise, they likely didn't bother hiding them at all, rather letting as many people see them as possible in the wake of the New York attack to give people a sense of security, knowing these things were up there to protect them.