The thing is many countries have their disaster management strategies for pandemics. But when it comes action, the strategiea fail. There are many reasons for this.
First of all, previous pandemics like bird flu had fast infection but it was known to the experts and easier than COVID-19 to handle. But COVID was unknown and it took more time to respond properly and actually there's still more unknown things that the responses cannot be validated. It's hard to respond unknown unknowns.[1]
Second, the strategies are great on paper but when it comes to implementing them, it is all about the politics. The politics of funding the research institutes and academic institutions, the structure of the nation's health care system, economic situation and use of state budget to support people in the economic crisis caused by the pandemic, the policies to allow human movement such as lock downs and restrictions... When it comes to actions, it's just the decision of the politicians.
Third, the strategies are created with stakeholders based on previous experience. And when they exercised, they are based on either previously occurred issues or predictions of the experts/participants of the strategy. You cannot be prepared for everything so you create many different feasible scenarios and exercise them. And they are limited by the capability of the participants.
Finally, strategies are already flexible because they are not binding for the states. A government can change the strategy on the go. A government can also totally ignore the current strategy for its political agenda.
[1] https://en.m.wikipedia.org/wiki/There_are_known_knowns