US rethoric says that they are concerned about IP theft, unfair economic practices, etc. But if you look at the sanctions then you see that they target pretty much only R&D-heavy sectors. That should be telling.
The semiconductor industry is extremely R&D heavy. You can't just steal your way into success. It is also heavily dependend on a global network of specialized suppliers and is one of the most complex supply chains on earth.
Even Chinese media says that they are not yet self sufficient. They predict that SMIC will become able to produce 28nm and above using only domestic inputs by the end of 2022, but that's just ability and doesn't factor in manufacturing scale to handle the corresponding demand.
Making 14nm fully domestic takes a bit longer -- maybe 2 years more. Chinese media and experts say that if they scale up 14nm and above, then that satisfies 70% of the demand.
To go further than about 7nm you need EUV lithography, which only has 1 supplier in the world, and the US is blocking that. How long it takes for China to independently develop EUV is unknown. Universities have made small breakthroughs in subfields here and there but they're still far away from the full solution.