The reason, of course, is so that the baby can be born in the US and thereby obtain the full rights of US citizenship.
So some of these Chinese engineers you speak of may be American born US citizens, although they’re wholly raised in and possibly completely loyal to China. The sad thing is that considering the culture someone was raised in (which seems possibly fair, though I’m not 100% certain it’s defensible) comes indistinguishably close to considering race (absolutely not fair in any way). The societal dialog exploring this distinction is fraught to the point that I am uncomfortable even making this comment, and I doubt HR departments are prepared to encode policies that are so perilous.
One escape hatch would be the US security clearance system, which is no doubt allowed to take many factors into account. But that wouldn’t apply here.
Why companies are still using engineers from China to work on trade secrets is beyond me. Even if they start off clean, they can be easily persuaded to steal by the Chinese companies or government.