No, you're not missing a thing! But, you obviously are a very self confident person and my primary point is that newbies may not feel so comfortable. Sadly, a lot of people aren't even sure what a cocktail dress is, let alone own one.
Honestly, I don't know very many ladies who, regardless of age, don't own either cocktail dress or a full length dress that they wear to weddings, to their company's Christmas party, and to other social events.
Also, if one does not understand the term "cocktail dress," it's not that difficult to take a walk through the local mall to see what the stores are selling as a cocktail dresses. It's also not that difficult to type "'cocktail dress'" into an Internet search engine -- which turns up plenty of pages loaded with pictures, like this one
I can certainly understand someone going on their first cruise not wanting to spend a lot of money on clothes they aren't sure they will ever use again.
That's right. Thats why guys who don't own suits typically rent tuxedos or dinner jackets. I really think that most ladies have something suitable in their closets, though -- as I said, they wear it to weddings, the company Christmas party, and other dressy social occasions.