The Austrian military didn’t just adopt LibreOffice; they actually contributed back to it. Over five person-years of development work went into adding features they needed. Those improvements are now available to everyone using LibreOffice, which is pretty cool.
You’re talking about a great number of organisations, with different decision makers. It takes time and political will to coordinate and execute this kind of big switch. This needs to happen to become independant from foreign monopolies, but I’m not surprised it hasn’t already happened.
The EU commission decides for some EU institutions. Member countries decide for their own institutions and military. Each country and military has its own labyrinth of bureaucracy with lengthy decision making, and large+complex IT infrastructures. All of this has inertia. And switching cost money, even if it’s possible to save on license cost on the long run.