A large part of that seems due to inertia; these kind of documents have always been typeset in a monospaced font, so there is bound to be a good reason for it — or so the mindset goes.

That reason usually boils down to 'because that's what it looked with typewriters before we switched to desktop publishing'.

In the US Courier stuck, in other countries it was Times New Roman with bad default choices for leading, font size, and line width. Legal and government typography for Latin alphabet based scripts seems to mostly be substandard everywhere.