Because you want to. :) Yeah, it's kind of sad that it doesn't have much more meaning than that, I know.
But also, it's not technically true that everything has been done over and over again, especially if you're not a hardcore creationist. For example? Humanity has existed for 200,000 years, but we've only organized ourselves into actual societies for the past 10,000 or so.
Before that, nobody grew any crops, nobody accumulated any wealth. There was no centralized leadership in the same sense as today, as people lived in small bands and they had to be nomadic, as resources for human beings don't generally all grow/exist in one area without us deliberately putting them there. There were no domesticated animals except for dogs.
The division of labor was not so specialized. Women tended to gather and rear children, while men tended to take on more physical tasks like hunting, fighting .etc., but for the most part people were their own doctors, they made their own clothing, they gathered and processed their own food, they made their own tools and so on. There was no shortcut, there was nobody you could pay to outsource these things. Either you knew how, or you died.
To put this all in perspective, imagine how different the world was only a few hundred years ago. Life in the 1800s was radically different from our lives today, yet even they were far more advanced than the majority of people that ever lived. They were innovative, cutting edge. Grug the caveman and his ilk would probably think they were gods, as would the souls who first transitioned into agriculture and sedentary society in the first place.
Our miserable asses today enjoy luxury and convenience the likes of which no other living creature, let alone our own ancestors, have ever seen or heard of.
And that's disregarding the fact that as far as we know, we're also the only ape lineage (or any animal in general) that's taken to such a radical lifestyle: we descend from herbivorous, tree dwelling apes, yet we somehow became spear chucking, music loving, culture-creating, language-having endurance predators.
That hadn't been done before in the entire history of the planet until we did it.
Yeah, unfortunately none of it has any greater meaning or purpose and it's depressing as all hell, so don't think I'm trying to invalidate you by saying all of this. I'm just offering a different perspective, I guess. Do what you will with it; I'm not judging you or trying to convince you to do anything in particular.
Personally, I don't think life would be much better even if it did have some kind of grand scheme behind it. Pain is pain even if you know why you're suffering, you know? The knowledge of some epic destiny beyond one's reach doesn't bring back lost relatives, it doesn't fix anhedonia. It'd be even more depressing, probably - you're not the "main character" after all. It'd be pretty lousy to know the purpose of humanity and life itself was actually to reach and colonize other galaxies one day and you're only a minor puzzle piece, if that makes sense.
It's probably better that it's all meaningless - this way, we can assign our own meaning and joy to things, if we so wish. We are all potentially as important as we want to be. You or I could up and decide at anytime that the purpose of life is to, I don't know, help others or some shit, and that would technically be true because we decided it was. Life could be about enjoying lots of good coffee. Itd be silly, but never technically incorrect.
