Because manufacturing super high tech chips is extremely hard and complicated and basically only a couple companies in the world can do it and they also depend on parts and technology from other companies all over the "western" world. China didnt have anything near this capability. It requires advanced research and knowhow on a million different technologies. So unless China does extensive industrial espionage(which they are trying to do), it's gonna take them like 10+ years to start producing anything relatively modern without needing any external parts/help.
In any case, it remains to be seen if it is actually 7nm. With a new cold war ramping up, China is trying to become self-reliant as much as possible.
Neither Europe nor the US can make chips like this, thats why they are trying so hard to convince TSMC to make a chip fab in their territories. Does this mean that white people are too stupid to make chips? No, they just cant atm and they arent willing to invest the infinite time and money it would take to reach that level, especially since their ally Taiwan(TSMC) is already making the most advanced chips in the world.
China on the other hand needs to become more self reliant, especially with the new Cold War escalating. So they have to do it. And they are doing it. But it will still take a lot of time and money to be able to manufacturer modern high end chips.
TSMC is taiwanese, which is "yellow" people. Why are you trying to spin this as a racist thing? Literally the best people in the world manufacturing chips are taiwanese(han). Why would anyone think that the chinese, who are basically the same ethnicity(han), wont be able to make advanced chips because of racial reasons?
Han Chinese 漢人/漢族 is the name of the ethnic group which originated from mainland China, and are the most dominant in mainland China, Hong Kong, Macau, and Taiwan.
OP here is probably trying to differentiate between Chinese (nationality) and Han Chinese (ethnicity) because although people in Taiwan are of Han Chinese ethnicity, they generally don't like to be called Chinese (nationality) and prefer to be called Taiwanese (nationality, though the official name of the country is Republic of China, but that's another story).
But it's not "yellow people," we also don't sell that tech to Russia, and they certainly aren't "yellow," and Korea and Taiwan are of the same general ethnic group as China, and they lead the world in chip manufacturing.
There's absolutely nothing racist going on here, it's just that everyone involved in core chip-making tech doesn't trust China. That's it, it's pretty simple. And China has earned that reputation by stealing IP from other countries, supporting repressive regimes (e.g. N. Korea), allying with aggressors (e.g. Russia), and bullying smaller countries (e.g. Taiwan, Philippines, etc).
It's not the Chinese people that anyone is worried about, it's the government of China. It really is that simple.
It's not stupidity. You need to invent the tools that are needed to make the tools that make the tools that make the tools that make the thing. China still hasn't independently invented most of those tools yet, so they've been relying on finished tools from other countries. Being smart doesn't wipe away the decades of research needed to make something. We all expect China to eventually be able to make an equivalent tool, but they're working with a handicap of inexperience which was expected to take years of dedication to overcome. Which is why everybody is skeptical that China made this independently without outside help or knowledge.