Why? Five very simple reasons:
1. I don't have to work within the same constraints that Apple did when they designed and built the machine; I can build something that's better suited to what I expect out of it than a regular G5 would be.
2. The G5 runs hysterically, laughably hot (not due to a flaw in the design; the G5's case and airflow setup probably has the best cooling potential of any consumer computer I've ever seen... the fan speeds just stay far too low, far too long) and the power supplies are apparently a problem. I can fix both of those issues by re-casing it, liquid cooling it and dropping a different form factor PSU in it that doesn't have to contend with dissipating as much as 130w of heat (assuming the dual 2.0s have a 650w psu, which as I recall they do, and assuming that the PSU is 80% efficient which is about midpack for most computer power supplies) in a case (the psu case, not the G5's case) that's very low profile and has two low speed fans that can't possibly be larger than 60mm to cool it.
3. Building it will keep me entertained for a weekend.
4. G5 systems are still far too expensive for me to buy simply to use as a toy (and let's face it, that's all I'm going to use it for; I have a G4, two G3s and 8 PCs in this room alone) It's a lot cheaper to buy the logic board and a couple of processors on eBay and deal with everything else myself.
and lastly,
5. It's a challenge. "What do I have to do to this machine to force it to do what I want it to do?"