Governments don't do get source code for the same reason as every other customer doesn't get source code: software vendors are incentivized to refuse the request.
Why are the vendors so incentivized? Well, coming back to Fujitsu and the Post Office, the answer is that refusing to share the source was worth about a billion dollars: https://www.bbc.co.uk/news/articles/cgm8lmz1xk1o