I'm not defending Alienware, but I see why people do. It's almost the same reason why businesses buy a Dell instead of OEM.
A lot of people:
- Don't want to be bothered with building the computer
- Learning cable management
- Troubleshooting problems
- Dealing with DOA or hardware that goes bad after a week
- Want a warranty for the entire system and not just one part within
For me, I love building computers. It seems like I spend more time working on improving my system and fixing it than actually gaming or something, but I enjoy doing these things. I like learning.
Some people just want to use their PC and really learn nothing from it, and they don't mind paying a premium for that.