Because most of the research is funded by tax dollars?
To be absolutely clear, I'm very much on the side of open publishing. I just want to be able to form a more accurate reasoning so that I can well explain it to others.
The government, as an economic actor, is entitled to attach whatever conditions it wants on its grants. The reason it doesn't attach conditions such as "you can't publish your results in <list of prestigious publications>" is because that would drive away the best and brightest applicants for the grants.
IANAL, but that's actually true. For example, government contractors are required to have affirmative action programs. See, for example here: https://www.shrm.org/resourcesandtools/tools-and-samples/hr-...