Because a century is 100 years and a foot is 12 inches. They are bigger quantities than the latter in each of your examples, and that is what you hear.
My guess is because the reference points used are not equitable. Even though the quantities come first (half of; 50), the brain can’t “do” anything with those quantities until it knows what they refer to. Therefore, since “century” is a longer measurement than “year” is, half centurysounds longer than 50 years.
I think it’s because we perceive 50 years as within the normal range of a human lifespan. And a full 100 years was something unattainable, a far-fetched dream. To live for a century or more is not something most humans are not used to—yet.