PS/2 ports are still there on high-end and "gamer" mobos.

This is also related to the fact that a lot of high-end keyboards/mouses are also PS/2.

They don't cost more than USB, are simpler in many ways, and if the mobo has dedicated ports for them, I think it make sense to plug the keyboard and mouse there to free up USB ports for other peripherals that are almost exclusively USB.