Hello,
Because we use bg for the slide itself https://prnt.sc/219hh6y and then if it switches from one to another slide then fade effect of the slider applied for the bg image https://prnt.sc/219hmg9
If you use just an image layer then apply the appearance effect for the image https://prnt.sc/219hqxt but slide switching won’t be so smoothy like with the image on the slide background.
Regards