Pausing a game is not as simple as freezing its entire state (and saving is also not as easy as dumping the entire game state to disc, at the least you'd end up with gigabyte-sized savegames).

Many game systems need to remain active in paused state (the UI needs to remain working for example, and actions in the UI may also manipulate game state (inventory, crafting, levelling up...). There are also plenty of games with 'active-pause' where the user can issue commands to units while in paused state.