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Because they are light and weak.
If you hit something hard enough to pinch flat a clincher, instead of killing the non existent tube you kill the tyre instead - the sidewall gets a cut as the rim slices through it.
You can pinch flat tubs this way too although it’s slightly less likely.
Amusingly I’ve never worn out a tubeless clincher - every single one has died this way, or the sidewall has worn out before the tread.
why?