Thrust depends on a lot of electrical and mechanical parameters, however, the manufacturer has no influence/say in the power system, and it is not their field of knowledge.
Specifying thrust as function of rpm hides/seperates those parameters nicely, just like with propellors. You don't specify an AC's cooling capacity by its wire diameter or the size of the holes in the mesh.
Having said that, I don't know why manufacturers don't specify EDF thrust/rpm numbers. Nah, for Chinese manufacturers it's probabably a simple explanation: the user is the tester.

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