As an attempt to answer the original question, I think there are three key reasons.
Firstly, they have to have something to fight against, because they have no positive policies of their own that people would rally round.
Secondly, they are, generally, trying to present themselves as being on the same side as people who are suffering from globalisation, neo-liberalism and the impacts of government decisions.
And thirdly, as has been pointed out, they don’t like any institution or tradition that prevents them indulging their authoritarian tendencies.
It is therefore both convenient and expedient to present those institutions as some form of elite, who, by doing tedious things like upholding the law, or insisting on political accountability, etc, are dragons themselves, or who are responsible for supporting whoever or whatever the populists have designated the enemy at any given time.