So, in an attempt to be a better person, or at least a less annoyed person (sometimes), I shall try to assume some things are due to lack of knowledge rather than intentional, um, ignorance.
New Adult is a weird term. I get that. There are places all across the web that will explain to you what it means, but my version is that it - like YA (young adult) or MG (middle grade) is an age category that references the approximate age of the protagonist(s). In the case of New Adult this range is 18-22, post-high school, possibly college, possibly not. So it encompasses folks more firmly establishing themselves, but still newly experienced at running their lives. Yes, like all age-based categories, there have always been books about people of this age. No, it doesn't have to be it's own category, but certainly one could understand why some people browsing might want a space to identify stories about that time of life.
It is not "sexed-up YA" as I keep seeing and hearing. First, because that seems to imply there is no sex in YA, which, is not always the case. (I hear teens have been having sex, well, since ever actually. Not all teens, but this is not a new development.) Second, this implies New Adult stories require sex, which again, no. Certainly, the number of gatekeepers involved with New Adult stories is probably* less. There are adult books that do not have sex, or have closed door sex, or well, every variation up to and including lots and lots of sex. So, while, yes, there is New Adult erotica, and I don't anticipate that happening on the YA front, that doesn't mean there aren't and won't continue to be New Adult sweet romance, New Adult historical, New Adult paranormal, and so on. So basically, now that the characters are all at the age of consent one expects a greater range of sexual choices, but that doesn't mean that New Adult books are only about sex.
*Yes, I realize this implies no one younger than that age reads up. I figure on or about the time people let you pick up adult or new adult stories, well, probably the gatekeepers are giving you free reign.