Because the job of a manager, that upon which managers are evaluated, is to change things.
So managers do not leave things alone: but change things.
And in changing things they wreck things.
Why do managers so reliably wreck things?
Here are some reasons:
1. Because of the decline in intelligence, the large majority of managers are heirs to organizations more complex than they can comprehend; hence they are (as a class) over-promoted and incompetent to maintain, repair and change institutions; plus there is dilution of talent due to the excess of managers, plus there is the deliberate dysfunctionality of appointing managers by affirmative action (i.e. on the basis of an inversion of group aptitudes).
2. Due to the principle of entropy, almost all possible changes are destructive; increasing of chaos, maladaptive, dysfunctional, efficiency-reducing - so only a few of the unbounded number of possible changes can, in principle, improve organizations.(Just as most genetic mutations are deleterious or fatal, and only few will enhance reproductive success.)
3. Managers are not competent to improve things (by their changes) unless they have specific relevant knowledge and experience of the core organizational function. But hardly any managers do.
4. Managers cannot improve organizations by changing them unless they are motivated to improve organizations - this is a personal thing: does the manager really rejoice in improvement, does he mourn a decline in functionality? To ask is to answer.
5. Managers live in a culture of dishonesty: half-truths, hype and spin. They lie to non-managers, to each other and (sooner rather than later) to themselves. Therefore, they do not change things such as to produce real world improvement, but such that they can plausibly claim improvement (to non-managers, to each other, and to themselves).