They would do that without hesitation. The problem (at the moment) is they have nowhere else to put that much money.
Their hesitation is they wouldn't get all their money back. If China pulled even 25% of its investments out of the U.S. the dollar would free fall. They wouldn't be able to cash out before most of the dollars value was inflated away.
China continues to buy our debt because they don't want the value of their current investment to collapse