Roughly speaking, an aircraft must fulfill a certain amount of economy (cheap, low cost to operate), safety, and performance.
If you compromise on safety, you get something that is still suitable for the military. If you don't care about economics you can participate in the space race.
But for commercial air travel, you don't have the luxury to pick just two; a competitive commercial airliner has to perform exceedingly well in all three regards.
If you're an airline using expensive aircraft you will go bankrupt. If your aircraft is too slow then your competitors will eat your lunch, and if you have a reputation of being unsafe then your customers will run away or the government will pull the plug (likely both).
IMHO affordable commercial air travel is one of the biggest marvels of 20th century engineering.
China currently can't make the high-performance, efficient, long-life jet engines that US & Europe make. The commercial market is heavily cost-sensitive, so they can't compete there currently as a result.
This doesn't matter so much for military purposes: they can easily eat the cost of a higher maintenance and replacement schedule on a smaller number of military jets with fewer hours on them.
This gives them more iteration cycles, speeding their building up of experience.
They're catching up. Industrial espionage will help them along too, but not as much as the experience from engineering their own designs.
OP: "So you'd think they'd be able to build a commercial jet liner, no?"
Sigh, the OP chucked out a casual insult at the Chinese because - unlike the USA and EU - their society is unable to (to date) build an international airliner business.
I deftly pointed out that the USA - despite is historical achievements - cannot build a high speed rail network and industry.
It’s would be a result of where the money and resources go, I assume. Apparently they haven’t felt a need to manufacture their own commercial jets but they did for military jets. They definitely feel the need in the case of chips.
