Why do apps rarely indicate the selected icon on their navigation bars

i'm looking for a suitable way to indicate that the user is on the cart page on an ecommmerce app by differentiating the cart icon from the rest of the icons on the navigation bar. there's obviously many way to do this (highlight, underline, darken the rest of the icon) but after checking out a few other apps noticed that except for a few most ecommerce apps don't do this. Any idea why?