Because oil need to penetrate very small niches inside engine. But size of these niches varies between different engines. Viscosity needs to be low enough to penetrate niche (not to stop at the edge) but high enough to stay inside and lubricate instead of just going through without much effect.
What's unalienable cannot be taken away or denied. Its most famous use is in the Declaration of Independence, which says people have unalienable rights of life, liberty, and the pursuit of happiness.
