The studies were done on cells (taking a sample from the surface after minutes or hours and trying to infect them - if they die, there is infectious virus there

The studies were done on cells (taking a sample from the surface after minutes or hours and trying to infect them - if they die, there is infectious virus there), but cells in culture don't have an immune system, nor they have physical barriers like nose and mouth. At this point animal studies are necessary to determine whether there is significant infection through items or not. Given the spread of the virus in the world, I think it is unlikely at this point, though (it would've been noticed if significant).