It's not actually that many people in the grand scheme at ~1.1 million according to the CRS, but it is spread out very effectively so that massive cuts can be directly felt by any chosen House representative or senator. It's partially a capacity preservation system as well, for defense a nation wants to be able to produce a significant portion of it's military hardware needs in a war domestically and you need to keep those industries alive somehow against cheaper foreign producers so you have the Berry Amendment that requires clothing etc purchased by the DoD to only use US made materials.