Because SMIC allegedly does not have enough production capacity
Despite the obvious successes of the Chinese company SMIC, which was able to create the 7-nanometer SoC Kirin 9000S for Huawei despite all sanctions, the US believes that sanctions are working well.
The point is that simply creating some kind of chip is not enough — it is necessary, among other things, to produce it in sufficient volumes. And this is what SMIC allegedly cannot do.
Neither the performance nor the yield can match the market performance of the device. Moreover, the chip that is inside this phone [Mate 60] performs worse than the one they had many years ago. Thus, our export controls are essential to slowing China's acquisition of advanced technology
Thea Kendler, Assistant Secretary for Export Administration, US
Unfortunately, Candler did not provide any exact data for the assessment, so it is unclear how insufficient SMIC performance really is for Huawei's needs.