Web browsers don't integrate to a single account and payment system, nor do they preemptively load entire websites before you start browsing. So you're always waiting for actions to complete or for images to load which feels slower. Mobile websites also tend to be very bloated slowing things down further than if the same functions were done natively in an app. There's also no consistency between websites so you never know when something will/won't work nor how far away you are from checkout. And then to top it all off there's browser compatibility, which is typically pretty poor for anything that isn't Chrome/Safari.

If a web browsers could really do the same thing all these companies wouldn't feel the need to make their own device specific apps.