Reasons: Addition reactions, detergents, and covalent carbon

(i) Unsaturated hydrocarbons contain double or triple bonds between carbon atoms which can break easily to add atoms (like $\text{H}_2$) to become saturated stable compounds. (ii) Detergents do not form insoluble precipitate (scum) with $\text{Ca}^{2+}$ and $\text{Mg}^{2+}$ ions present in hard water, unlike soaps. (iii) Carbon has 4 valence electrons. It cannot gain 4 electrons ($ext{C}^{4-}$) due to energy constraints or lose 4 electrons ($ext{C}^{4+}$) requiring extremely high energy, so it achieves octet by sharing electrons forming covalent bonds.