are there any technical reasons as to why i shouldn't...
basically we established that it is technically possible. The problem is that there really is no margin for error. If you make mistake, cross a wire, something accidentally cuts through the wire insulation, or something completely unforseen happens( Murphy's Law), you have 100 amps going straight to ground, (that's called welding). At best- the computer crashes, at worst- sparks, fire, and the release of very expensive magic smoke. Other dangers include dumping 12 volts unto the 5volt or 3.3volt rail (= fire, sparks, magic smoke released), or introducing nasty transients onto your computer's power rails( crashing, blue screens, and unhappy computer/magic smoke).
That said, other technical considerations include: decoupling, circuit protection, and load currents. Most PSUs either have 18awg or 16awg wire which really isn't suitable for large currents. Have you calculated your total load including what your computer components draw as well as your project?
It just makes more sense to buy a $20 15amp 12volt supply off ebay than to risk a computer that costs exponentially more.