It does not have to be. The English language has a process where phrases become hyphenated compounds which then become single words. It's permissible to be partway along that path, and for people to disagree where something is on that path.

Pick any point in the past few centuries, and there's going to be something, possibly nowadays always a single word, but not necessarily so even now, that was in a state of flux at the time. The same goes for today.

Have a nice week-end! I fondly recall the days of my youth as a teen-ager.

Though I still lament the loss of the subjunctive form. And diaeresis.

Tangential, how do you hyphenate (((very old) growth) tree)?