It doesn't, Debian testing and unstable are rolling release, and they package -dev separately. Lack of separate -dev packages is something I like very much abo

It doesn't, Debian testing and unstable are rolling release, and they package -dev separately. Lack of separate -dev packages is something I like very much about Arch because I build enough software that I'm happy to pay the storage cost for the headers in exchange for not having to track them down from the eventual compilation errors. It is entirely justified for a distro targeting a demographic that builds less, or with less available storage, to have separate -dev packages.