Given the dod budget is bigger than the revenue of google, amzn, and msft combined and with a million employees why can't they just build their own data centers?
If I had to guess, it is because the working culture of public organizations (often a lack of motivation/incentive due to lack of competition), combined with tenure-based job security, and artificial constraints (pay structures/levels that don't align with the competitive market) mean that someone like the DoD can't really get something like this done. They wouldn't be able to hire the right talent. If they did hire talent, that talent would need to work in the shadow of existing hierarchies/authorities that aren't suited for the role. Not to mention that bidding processes like LPTA (Lowest Price Technically Acceptable) would hinder procurement with a lot of red tape and poor decision making.
TLDR, because they aren't set up for success like private organizations.