Why Geography is a field of study of which time in general steps into its background?

because the timescales involved are so unimaginably large. Everything on the planet (physical features and terrain) is either being eroded or built up (deposited), on a microscopic level constantly for all of earths history and future. some things are measureable in a humans lifetime like coastal erosion for example, but most are not.

Mountains didnt just pop up one day and stay there, they are slowly being eroded back down to size and one day will be flat. basically the earth wants to be flat, but pressure builds deep down in the core, things crash into each other, and as newtons laws of motion state for every action there is an equal and opposite re-action.