What are romaine hearts?

So I’ve been musing about romaine hearts and this is what I’ve noticed. They are a pale green, unlike full romaine heads, where the outer leaves are a rich green. This led me to wonder, why this product exists. Where did the darker green outer leaves go? As lettuce matures from the outside in, those outer leaves would contain the most nutrients, yet they aren’t in romaine hearts. Either they spoil too bad, the heads are picked before the dark outer leaves mature, or they are removed and sold for something else. Maybe for chopped bagged romaine lettuce at a premium? Chopped and washed sells for around $4 for 255gms, and hearts for about $4 for 1.5kg. Now I understand that washed and chopped is a premium product, but equivalent weight, that’s $24 for 1.5kg. I wonder how nutritionally valuable romaine hearts really are, since they have not reached the maturity that the outer leaves have, that have been stripped away.