It's not, though. And it's not just physics.
The notion that natural phenomena are working in the same way they did in the past as they did today, and will continue in the future, refers to all the Earth's mechanisms. Gravity for example, has always been gravity...there may have been changes in how gravity was experienced on the planet (i.e., during the formation of the Solar System, with orbital and lunar paths being closer and/or further than they are now) but it was still gravity acting like gravity does. Other examples would be tectonics (and the rock-cycle), carbon cycle, hydrological cycle, isotopic decay....all of these things are constant in the nature of how they work.