Tar remains flexible because it contains oily, resinous, and heavy organic compounds that do not all harden into a brittle film.
Flexibility is one of tar’s useful traits. Wood, rope, leather, and boats move. They swell, shrink, twist, bend, and vibrate. A coating that becomes too rigid can crack quickly on those materials. Tar can move with them more than many brittle finishes.
This flexibility comes from the mixture. Tar is not a pure single compound. It contains heavier fractions that remain plastic, especially away from the surface. The outer layer may weather and firm up, while the inner material stays more accommodating.
Heat increases the flexible character. In warm weather, tar softens and moves more easily. In cold weather, it stiffens but may not become glass-hard unless the product or residue is already highly aged, thin, or modified.
The benefit of flexibility is durability on moving surfaces. The drawback is that flexible tar can stay tacky, hold dust, mark clothing, or creep where a clean hard film would be preferred. It is a trade-off, not a flaw by itself.