You can't step in the same river twice because change is constant and all around us. If you step into the Mississippi once and come out, by the time you step back in the river has flowed more and the river has changed. To be more specific, rocks in the river are no longer in the same spots, the fish that were there earlier when you stepped in have swan up the river, and the footprints that were left by you in the river are already disappearing by the time you decide to step back in. Another thing that supports this idea is the fact that the earth itself is in constant flux, continents are always shifting, rivers are moving, everything on earth is changing constantly which is all invisible to the naked eye. In fact field's like physics, and chemistry which study the movements of particles, objects, and energy, without change, these subjects wouldn't exist. Finally, I think that time is another factor that can be considered when asking this question. When you are stepping into the river and then stepping out, that moment in time is lost forever thus you can't go back to redo that moment. The