We choose to do costumes from scratch, because then they are unique. You can be sure that no one else have made it like you have and you even might have done little better than some other cosplayer. Little competition is healthy, I'd say. People do get their costumes bought and it's cool, but we just can't get enjoyment out of that. It's much more satisfying to do it yourself than boss around wearing something that anybody could have.
I see cosplay purely fandom based. Wearing a costume is a sign that you like the series you are cosplaying from. I can't see the connection when you buy costume from company that benefits from your interests when it comes to your hobby. After all you like the series, you aren't the fan of the costume making company. I don't know do the makers of manga or anime get their share when costumes are bought, that is a matter I should see into before I start pointing fingers... But still, cosplaying is your own choice to display your liking and making your own costume is a sign of your devotion.
Then there is the accuracy aspect. Like all mass producted clothes, costumes are also made based on basic figures. They might not suit your bodytype or proportion. When you make a costume you can affect on measurements. Right cut can work wonders even if you don't have the exact figure of your character. Sometimes ready-made costumes lack details. When you work from your reference pictures you can get every little thing fall to it's place. And we do take reference pictures seriously. It would have been really easy to buy hitai-ate for Deidara, but they just are so ridiculously big and not in any scale compared to human that I just couldn't approve it.
Still some bending with the references is good or even needed, because sometimes characters designs just miss seams that are necessary to make a fitting clothes.
Of course there are some things that are really hard to do. Wigs are the possibly the most usual objects people purchase for their costumes. High-quality wigs which are styled according to the characters could be a possible choice for us, if we were to cosplay character who has ridiculously long hair etc. We have this far dyed and styled our wigs by ourselves and it really gets easier every time.
But then there is also things that you can't find anywhere. For example Mayuri's hat, ear-chin piece, detonator and... uh... face. When you pursue accuracy you just have to make the work yourself... Or you can commission it. I see nothing wrong in commissioning props or wigs if you give respect whom ever it belongs to. And this I could say about bought costumes too which are purcahed from costume-makers or possibly sewing skilled friends.
There are lot of great tutorials when it comes to making a costume. They are not too hard to find and usually fellow cosplayers gladly answer questions. I recommend that if you have seen some costume which you are going to make and like the result go and ask directly from the cosplayer how it was made and what materials were used. Of course some people wan't to keep their secrets for themselves. If you don't get an answer try someone else (in our case, be patient... sometimes we just are slow or suffer memory loss). Search for forum posts or make one yourself.
As I said before, I don't think ill of people who buy their costumes. I just give respect to them who makes the effort. Crafting skills are not self-evidence but with imagination and guts you can work wonders.