Mainly because they are absolutely clueless about the physical world (and by observation not too flash when it comes to the financial one either)
Moreover, they mistake carbon dioxide emissions as costs rather than the tremendous good they are for both humanity (boosting crop growth) and wildlife (increasing primary productivity and protecting more wild lands from the plow). Since they start from such a flawed premise they have no hope of ever getting the economics of energy and society anywhere within the ballpark.
When free markets do not maximise society’s welfare, they are said to ‘fail’ and policy intervention may be needed to correct them. Many economists have described climate change as an example of a market failure – though in fact a number of distinct market failures have been identified.
The core one is the so-called ‘greenhouse-gas externality’. Greenhouse gas emissions are a side-effect of economically valuable activities. Most of the impacts of emissions do not fall on those conducting the activities – instead they fall on future generations or people living in developing countries, for example – so those responsible for the emissions do not pay the cost. The adverse effects of greenhouse gases are therefore ‘external’ to the market, which means there is usually only an ethical – rather than an economic – incentive for businesses and consumers to reduce their emissions. As a result, the market fails by over-producing greenhouse gases.