Because I'm totally anal about my arrows...especially carbons.

IME carbons need to be cut from both ends to get the straightest shaft. Then I grind down the ends square to make sure components will fit as straight as possible.

I also like my fletches to be a bit further up the shaft than most people fletch them. Keeps them from making contact with my face/goat-tee.

Plus I like to experiement for best spine & FOC along with different degrees of helical/offset, size of vanes/feathers, and different vane types.

And it's fun.

Once in great while I'll buy shop or factory fletched but it's purely a convenience thing. My shop does a pretty good job w/ fletching but I still prefer to "roll my own"