How come already built PCs like iBUYPOWER Desktopsmare so cheap compared to building one?

Solution
The parts are typically a lesser quality for those pre-built machines. Here is the ibuypower pc if you built it yourself (or close enough).
PCPartPicker part list: http://pcpartpicker.com/p/YKwZzy
Price breakdown by merchant: http://pcpartpicker.com/p/YKwZzy/by_merchant/

CPU: Intel Core i7-4770 3.4GHz Quad-Core Processor ($294.99 @ NCIX US)
Motherboard: ASRock H81 Pro BTC ATX LGA1150 Motherboard ($44.00 @ Newegg)
Memory: Kingston Blu 16GB (2 x 8GB) DDR3-1333 Memory ($138.99 @ Newegg)
Storage: Toshiba 2TB 3.5" 7200RPM Internal Hard Drive ($74.99 @ Micro Center)
Video Card: EVGA GeForce GTX 650 2GB Video Card ($99.99 @ Newegg)
Case: Diablotek DIAMOND ATX Mid Tower Case w/400W Power Supply ($29.99 @ Micro Center)
Optical...
That build has this graphics card;

NVIDIA GeForce GTX650 Graphics With 2GB dedicated graphics memory

Also;

Terrible and cheap power supplies included
You'll find there'll be a very cheap motherboard in there
HDD's (2TB) aren't very expensive

For $1000, you can get a far better card then that, with an i7 4th gen also.

You could buy a similar system for under that if you include a cheap PSU and cheap motherboard with same specs. We could make you up a better parts list if you want for $1000 if you link up preferred sites.
 
Here's what I'd do for $1k:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i5-4460 3.2GHz Quad-Core Processor ($184.99 @ NCIX US)
Motherboard: ASRock H97 PRO4 ATX LGA1150 Motherboard ($76.66 @ Newegg)
Memory: A-Data XPG V2 8GB (2 x 4GB) DDR3-1866 Memory ($69.30 @ Newegg)
Storage: Crucial MX100 256GB 2.5" Solid State Drive ($106.81 @ NCIX US)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($51.99 @ Amazon)
Video Card: Sapphire Radeon R9 280X 3GB Dual-X Video Card ($279.99 @ NCIX US)
Case: Antec NSK4100 ATX Mid Tower Case ($46.99 @ Mwave)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply ($45.99 @ NCIX US)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Total: $967.69
Prices include shipping, taxes, and discounts when available

Note that theirs has a GTX650.
 
+1 to above; R9 280X vs GTX 650, there isn't one, as the R9 280X is far better, even with the above build having an i5 and the pre-built with an i7, for gaming you'll find no excessive difference between them, but with the far better card, you certainly will find the above rig far better, and the power supply is also of far better quality.
 

NBSN

Admirable
The parts are typically a lesser quality for those pre-built machines. Here is the ibuypower pc if you built it yourself (or close enough).
PCPartPicker part list: http://pcpartpicker.com/p/YKwZzy
Price breakdown by merchant: http://pcpartpicker.com/p/YKwZzy/by_merchant/

CPU: Intel Core i7-4770 3.4GHz Quad-Core Processor ($294.99 @ NCIX US)
Motherboard: ASRock H81 Pro BTC ATX LGA1150 Motherboard ($44.00 @ Newegg)
Memory: Kingston Blu 16GB (2 x 8GB) DDR3-1333 Memory ($138.99 @ Newegg)
Storage: Toshiba 2TB 3.5" 7200RPM Internal Hard Drive ($74.99 @ Micro Center)
Video Card: EVGA GeForce GTX 650 2GB Video Card ($99.99 @ Newegg)
Case: Diablotek DIAMOND ATX Mid Tower Case w/400W Power Supply ($29.99 @ Micro Center)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Total: $787.92
Prices include shipping, taxes, and discounts when available

See? For less than $800 including the operating system, you can have the same thing, and that is without going super cheap. Or you could go for something in the same price range like this:

PCPartPicker part list: http://pcpartpicker.com/p/4yh84D
Price breakdown by merchant: http://pcpartpicker.com/p/4yh84D/by_merchant/

CPU: Intel Core i5-3470 3.2GHz Quad-Core Processor ($189.18 @ SuperBiiz)
Motherboard: ASRock B75M-DGS R2.0 Micro ATX LGA1155 Motherboard ($52.89 @ Amazon)
Memory: G.Skill Ares Series 16GB (2 x 8GB) DDR3-1866 Memory ($139.50 @ Newegg)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($51.99 @ Amazon)
Video Card: EVGA GeForce GTX 770 2GB Dual Superclocked ACX Video Card ($329.99 @ Amazon)
Case: Rosewill Line Glow ATX Mid Tower Case ($39.99 @ Newegg)
Power Supply: Corsair CX 600W 80+ Bronze Certified Semi-Modular ATX Power Supply ($49.99 @ Newegg)
Optical Drive: Samsung SH-224DB/BEBE DVD/CD Writer ($14.99 @ Newegg)
Operating System: Microsoft Windows 8.1 (OEM) (64-bit) ($89.98 @ OutletPC)
Total: $958.50
Prices include shipping, taxes, and discounts when available
 
Solution