Why does "half a century" sound longer than "50 years"?

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.

Well, I much prefer to say I am 50, than I am a half century! Sheesh….lol

lol

Four decades plus nine?

Two quarter centuries even?

For the same reason ‘four score and seven’ sounds better than 87. It is exotic and therefore has more resonance.

—It is also why I can go into my local McD and get 6 chicken nuggets but not half a dozen—

Not to me.

More syllables.
I didn’t ever learn to count them properly. I think it’s 5 syllables versus 3.

Because hearing the word century puts the number 100 in your mind.

But half a century is longer than fifty years. Anyone knows that! :-J

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 century sounds 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.

They sound the same to me.

Because the word “century” sounds longer/greater than the word “years”.

Because a century sounds alot like a millenia (too long for most people to relate to).