Why some of the plastic parts have to be made by CNC machining instead of the injection moulding?

1.Some of the plastic material don’t have injection grade material, for example: PTFE and UHMWPE, the viscosity of PTFE is very high, so it’s very difficult for the melt PTFE and UHMWPE flow and full fill the cavity.
2.In order to meet the industry design requirements, for the plastic parts, there’s some feature which is not feasible for injection moulding, for example: the injection moulding parts have to have draft angle, except that the injection moulding plastic parts have to had better have even wall thickness, however the CNC machined plastic parts don’t have these limitation.