For the same reason that China did not need conscription when it fought the Korean war. It will have no problem recruiting enough volunteers, just like the US had no problem recruiting enough volunteers post-Vietnam.
And because any conflict its likely to fight will not be one across a two-thousand kilometer front that physically requires millions of people sitting in trenches. It's not 1943 or 1917 anymore.
There are more people between the ages of 15 and 35 in China than there are people alive in the United States.
There are more 20 year-olds in China than there are men in Taiwan.
You don't need conscription to fight a naval war when you have almost a billion and a half people. Your demographics support finding enough volunteers.
Also, the one-child policy put China's fertility rate at 1.70 children per woman. You know what the US fertility rate is?
What do you mean? Sure it has a large population, but it's also a nation of only children.