Because with a refurb you stand a better than even chance that there will be a problem, and with Dell's the odds go up even higher. A hobbyist will be able to diagnose the problem quite easily, but most mother-in-laws are neophytes and can get frustrated almost immediately. In addition, Dell's customer service is a horrifying experience for people who don't know computers at all (and fairly miserable for those of us who do...).
For about the same money, you can something like the following from Newegg or most other places:
AOpen ES50A Black SGCC Steel ATX Mid Tower Computer Case 300W Peak Power Supply - Retail
$32.99
ASRock K8NF4G-SATA2 Socket 754 NVIDIA GeForce 6100 Micro ATX AMD Motherboard - Retail
Model #: K8NF4G-SATA2
$60.99
AMD Sempron 64 3000+ Palermo 800MHz FSB Socket 754 Processor Model SDA3000BXBOX - Retail
Model #: SDA3000BXBOX
$87.00
CORSAIR ValueSelect 1GB 184-Pin DDR SDRAM DDR 400 (PC 3200) Unbuffered System Memory Model VS1GB400C3 - Retail
Model #: VS1GB400C3
$84.55
HITACHI Deskstar 7K80 HDS728080PLA380 -0A30356 80GB 7200 RPM SATA 3.0Gb/s Hard Drive - OEM
Model #: HDS728080PLA380
$54.00
Microsoft Comfort Curve 2000 B2L-00047 Black Ergonomics Keyboard Mouse Included - OEM
$23.00
Acer AL1714CB-8 Black 17? 8ms LCD Monitor - Retail Model #: ET.L1809.102 ** This item is warranted through the product manufacturer only. **Dead Pixels Policy: Replacement or Refund for 8 or more dead pixels only.
$205.99
LITE-ON Black IDE Combo Drive Model SOHC-5236V BK RTL - Retail
Model #: SOHC-5236V BK RTL
$29.99
Subtotal: $578.51
This is a faster, more stable system...and all of the parts are new.
I do think you should get them to build it though, as that way they will be supplying customer support instead of you...
🙂