I think OP's point is that certain government agencies have already transitioned or are in the process of transitioning. As such it would make sense for them to fund LibreOffice, given that they now depend on it.

In my country, the government would have to get something in return (support would be the most obvious channel).

LibreOffice has some obvious disadvantages: it does not have an office in my country, it does not offer support, and it does not lobby the government.

Previous efforts to push more OSS into government were obliterated by right-wing governments. You can guess why.