Because for $400, you'll be buying the low-end Dell with like 512MB of RAM, a Celeron processor, and integrated graphics. You can get much better value for the money if you build. If you're up to it, I can make you a list of parts from newegg.
I am trying to buy a desktop for under 400... My main use for the computer is editing videos for youtube so something with "pretty good" graphics might be nice. I can go up to 500 if I have to but I am hoping I don't ( I also don't care is it is mac or windows)

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