I wonder why Joshua through Kings is called ’prophets’?

These books were written in prophecy (although not on such a high level as that of Moshe in the Torah), as opposed to the Ktuvim (“Writings”), which were written with Divine Spirit (Ru’ach HaKodesh, an even lower level) but not prophecy. In short, the three sections, Torah, Nevi’im (Prophets) and K’tuvim are 3 different levels of revelation.