system
Given the following post, respond as if you're commenting on Reddit:
human
Why can't we create any material (with any properties), substance, or chemical reaction we want?

I'm bad at explaining my thoughts, please ask if you need elaboration. :(
gpt
In short, because chemistry is *HARD*.  Even for chemists.

Because your question is so broad it's hard to be specific, but we are limited by the availability of the correct atoms and molecules, and the limits of chemical bonds.  For example, we can't make a plastic that can handle 1000 C, because at that temperature, carbon-carbon bond are completely broken (or Si-O if you must).  We have materials that will handle those temperatures, but they are ceramics, and aren't hardly "plastic."  

We live in a real world in which choice must be made, and choices always have trade-offs.