Why are there so many weird population projections that the westerners are obsessed with?

It’s even more notable when it comes to China. I’ve seen wild estimates of China falling to 800, 700, 600, 500 and EVEN 300 million people by the end of the century from the current 1.44B! And their only justification for these numbers are that Chinese govt made the current number up for some reason that they never give other than because for some yellow peril reasons.

Who’re these people that’s making up all these wild scenarios? Are they legitimately demographers or just libs who just want to see China do bad so they can justify an invasion?