Because it's not that much effort? I also didn't change it all to dead stock... I spent a long time weighing up the positives and negatives. Consider this. I paid 15k for my car when I bought it. At the time I did the conversion last year, there is no chance I would have got 13k for it. The conversion including all parts (necessary parts, not optional parts that I chose to use) cost me about 6-7k. So by the time my car was essentially a GTT, perhaps I could have sold my car and bought a car worth 20k.
So yes, perhaps I could have bought a factory GTT. And yet for the same price, I got so much more. Even if you ignore all the parts that I upgraded (too many to list off the top of my head), the experience was priceless. I have done too much to my car to consider owning a different one. If I was to sell my car, it would only be to move away from the high performance scene.
I agree a build is the learning and the "experience". Having selected, "held", been involved with your cars performance parts, is something I find more interesting than just trading cars and retail therapy.