|laziness, impatience, and hubris|
I didn't see scratchbuilt as a choice on that poll.
My first machine was built up from an 8085 chip and its friends on a perf board and wire wrapped. It wasn't even an S100 bus machine.
Later on I inherited an Altair that someone had given up on assembling. After many hours of troubleshooting and eliminating his cold solder joints I had a working machine.
Next on the parade of machines that trapsed through my house was a VIC-20 that I interfaced to an ASR-33 teletype so that I would have a printer to use.
Next came not one but two C-64s that had short lives.
After that I bought my first AT clone and had a ball messing with it until I finally bought a 386/33 machine that had me thinking "this is it, things will never get better than this!"
Nowadays I have a 2.2Ghz ASUS based system that I built up from parts, a SS-20, and a myriad of refugees from junk shops at home.
I haven't changed a bit. I'm still hacking at the hardware.