There is no one reason. But I will try and give you some. The first one is necessity. Stuff wears out or breaks, might as well replace it with like or better.
The second one is progress. It might be easy to be all curmudgeonly and cynical and say that companies are just out to rip us off but compare a bike from now and from 5 years ago (go ten years back if you’re not convinced). Compare a top of the line Spesh Enduro from then and now. Yes the old bike was, and still is great, but the new one is better in every single way. Compare brakes, forks, shocks, carbon components…it’s all better. You can’t argue with that.
And lastly there is just the lust factor. I love bikes, I like bike parts, I like building bikes, having a nice new bike…etc etc etc. So if I can go hungry for a few months to get one, why not!