Because they "believe" in it, but have demonstrated *many* times over my lifetime that they don't understand it.
Random case in point. Take one electricity generation industry. Impose a carbon tax on nuclear plants, but fail to require carbon capture on fossil plants, all in the name of global warming. Then express surprise that the only economic technology is the one you've implicitly subsidised to the tune of several billion per year. Then force all providers to buy back outrageously overpriced electricity from small-holders, whether they want it or not, and express utter astonishment that this cost is simply passed on to customers.
Random case in point. Take one railway, break it into a dozen pieces, write contracts that offer guaranteed subsidies and no penalties for crap performance. Then wait. Then express surprise that all the incumbent providers are crap and rich.
Random case in point. Take one education system. Farm out the task of setting exams to private bodies, who then compete for schools' business by offering "more attractive" exams. Then express surprise that results go up year on year for a couple of decades whilst teachers are invited to seminars where they can learn what might come up in the exam.
Random case in point. Having inherited a banking system that collapsed because there were no penalties for betting someone else's farm on a lame horse, bail out the losing bankers but offer no comparable help to the prudent ones who are still solvent. Then express surprise that you're broke and the banking system is still paying out huge bonuses to the same idiots as before for taking the same risks.
I'll stop there, except to re-iterate that for a party which claims to believe in the free market, they haven't a clue about the importance of letting losers lose, the importance of maintaining a level playing field, and the phenomenon of getting *only* and *exactly* what you ask for.