It's complicated. The Army Corps of Engineers has had a civilian mandate to support flood control prevention since 1917 [1]. Beyond that, they are also involved in large public works projects such as the building of roads and bridges, and Superfund clean-up sites. On top of this, they regularly receive large pork barrel grants from Congress that can siphon money into a senator's state or a congressperson's district. They do have a large contracting arm and are actually pretty well-regarded for their comprehensive procurement and management process for these large public works projects.

So it's scale, politics, and history/momentum at this point.

[1] https://en.wikipedia.org/wiki/U.S._Army_Corps_of_Engineers_c...