To be clear, I’m not asking how to repair it, I’m just trying to understand in general terms why it is so difficult to repair effectively when on paper it seems like typically an easier material to work with than wood or metal (both of which are fairly trivial to repair).
Flagging this post as chemistry because I suspect that’s part of the answer, but it might very well be a physics problem instead.
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Plastic is made from, and gets many of its useful properties, from the long chains of polymers that are knotted together. Polymers are just long, repeating molecules. Plastics are flexible because the chains aren’t often chemically attached to each other and can slide around, but are still strong because they are still physically intertwined (like thread in cloth but more knot-like).
When you damage plastic, you’re damaging those chemical bonds, creating lots of loose ends. Using heat or chemical agents to re-bond it is effectively just mashing those ends back together and hoping they get tangled enough to hold or fusing them mostly at random. Even if you do join the parts back together, the fused area is often stiffer, weaker, and more brittle than the surrounding material, and thus is prone to breaking and snapping again. It’s also very difficult to make visually seamless repairs, so in places where strength isn’t a factor its still awkward.
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