Simple, because it's so damn obvious from the intro. If you played FO3 you would immediately notice it. And because i spent hours on the game and barely did any of the main story. The crafting, settle ment building, and exploring will take up like 80% of your time.
Listen to Gunzy, bro (I assume you're a guy?). I don't do Fallout because of the action, but for the exploration. It being a shooter is just a huge bonus.
Because you don't know many of the side quests and the outcome of them. Because there are many, many stories to learn about throughout the game that has nothing to do with the main quest. And because exploration and interactions are worth even more than the main quest.
And because it's fun to blow heads off now and then.