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Why is heat harder to convert completely into work than mechanical energy?

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Study how a series of gears in a waterwheel translates a stream's energy to a millstone — energy conversion Before the Industrial Revolution, power came from three main sources: humans, draft animals, and water.
The ingenuity people used in harnessing waterpower can be seen in this medieval-style mill. The waterwheel is turned by a stream and is connected to a shaft that leads into the building. At the other end of the shaft is a gear. The connection of a series of gears translates the power from the stream to a shaft that drives a millstone, which grinds flour from grain.
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The difficulty in converting heat completely into work, compared to mechanical energy, arises from the fundamental nature of thermal energy and the second law of thermodynamics.1

While forms of energy like mechanical energy involve ordered motion that can theoretically be converted entirely into other forms, thermal energy is associated with the random, disordered motions of a material's constituent parts. Because work requires directed energy, this inherent molecular disorder—quantified as entropy—means that a portion of thermal energy is always unavailable for conversion into useful work.12

The second law of thermodynamics dictates that a heat engine must exhaust some heat to a lower-temperature reservoir to operate in a cycle. The maximum possible efficiency is limited by the temperatures of the heat source and the sink, as defined by the Carnot cycle. Consequently, achieving 100 percent efficiency is physically impossible, as some energy must always be dissipated as waste heat to satisfy the requirement that total entropy does not decrease.3241