This post from a previous discussion should help answer part of your question.
For the rest of the specifications, it is due to the major differences in the hardware architecture between the PS2 and an x86 32-bit processor from Intel or AMD. In short, each difference has to be emulated, and there is generally no shortcut for these. Each modification can have drastic results, and the point in emulation is for accuracy and not for speed. Just like many complex products, there are always pieces that can be done much more easily with the actual hardware than by software emulation on a different platform: some methods and tools just do not work properly under certain conditions. (In part, this is due to the computer processor and graphic card manufacturers, and even the manufacturer of the big OS components, Microsoft.)
Keep in mind the increasing complexity of console devices. In the days of the
PSX, the internal hardware specifications were low and "simple" enough to be emulated by a mid-range PC today. However, consider the
PS2's, the differences are extreme.
Even the slightest differences can cause headaches, but considering these huge exertion, it will take some time and a lot of horsepower to pull it off.
Hopefully, this answers most of your questions.