They don't mention the type and brand of Motherboard, PSU, case, memory, HDD.
They could easily throw some uber-cheap parts in there. Making the whole build crap.
By building yourself, you know exactly what's in there, and can customize it to your needs.
The graphics card for example included in this 1100 build is way weaker then the graphics cards a gamer can put in his custom gaming build for 1100$.
It's easy to say "xx GB ram included", but you have poor quality rubbish RAM and you have the real deal.