In most public institutions performance doesn't impact salaries. Strategic planning involves significant research and effort, and managers do not see the need for additional engagement.
There are several factors that can influence in this question, in addition to the previous comments. a) The knowledge about the instrument and its development; b) The lack of a political and institutional structure that promotes, supports, implements and evaluates. This can allow the stability, continuity and projection the strategic plan, even if managers have to invest time; c) the importance given and the knowledge by politicians of this instruments. Despite the effort, they are looking for a rewarded in votes, but not always have them.
For this interesting question there are various failure factors. I will mention some very relevant ones:
First, an strategic plan for public organizations must start from a sound up to date evaluation of its role, competence, relevance, capacities and sustainability for the medium to long term.
Second, the organizational strategic planning needs to involve a highly coordinated, efficient and effective structure and operation capacity to implement their specific services innovation with a permanent public assessment
Third, the formulation of the organizational strategic plan, should not be done with selected key internal actors, but involving all those who work in the organization and most of all, as a result of a careful identification of specific demands, pros and cons of the organizational work, extracted from the very perceptions and complains of citizens and clients.
In Brazil, the organisations have a big influence of politics that difficult the policies implementation based on strategic plans. Also, there is a lack of monitoring tools inside the teams to aliign the strategic plans with the programs and actions.
You may find it useful to look at a very recent report by the Australian National Audit Office on 'Corporate Planning in the Australian Public Sector' at (https://www.anao.gov.au/work/performance-audit/corporate-planning-australian-public-sector-2016-17). Appendix 3 in particular provides some good insights on why strategic planning fails, by setting out the factors to be considered such as
whether and how the plan is positioned in the organisation and more broadly with government and non-government stakeholders
systems and processes in place for developing the plan
I would also add lack of "real" participatory processes, in which citizens and stakeholders are engaged from the very begining of the planning process until its end. This lack of participation usually implies poor analysis of alternative options and poor monitoring.
Corruption might also be an answer, since powerholders exert their power during both phases ex-ante and ex-post in order to shape planning processes.