Scientism and the cult of the expert are fundamental features of contemporary liberalism.
Liberalism intends to abolish oppression. Since oppression includes all subjection to the arbitrary will of another, liberalism attempts to base government action not on will, or on particular personal understandings of the good, but on what the situation objectively requires as determined by a rational and impersonal public process—in other words, by trained and credentialed expertise. Further, the liberal understanding of oppression expands, in accordance with its tendency toward abstraction and universality, to include all personal and social inequalities, however informal or even natural. Abolition of oppression therefore comes to require comprehensive control of all social relations, which again must be passed off as the impersonal rational requirement of technical considerations and of human rights, both as determined by experts insulated from political pressure.
The ultimate consequence is that liberal societies tend to become despotisms comprehensively controlled by elites responsible only to their own understanding of what needs to be done. The ruling elites of course deny that anything of the sort is going on, since doing otherwise would destroy the basis of their claim to rightful authority—the prevention of oppression.