They are expecting a war. They have poked the bear one too many times.
They are scared shitless of an armed populace. As they should be.
They are full of easily-hacked software, otherwise-unemployable drones, and lots of conference rooms.
They are also often located in the least desirable parts of town, so they're not particularly useful real estate for the most part.
If I were a revolutionary strategist I would simply ignore federal buildings, not target them.