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.

The initial roll out of USB mice was on 125Hz polling interval. The PS/2 mice were faster at first because they sent the signal immediately with no delay.

You can easily see the difference - with 125Hz it's impossible to draw a good circle with a mouse since it will always draw short straight lines