Google is a US company but the "welfare loss" its monopoly create is on the whole (western) world and not just the US economy. That is why it won't be regulated out of existance.
It is better for (US) national security that Google keeps its (global) monopoly. (Non-US) competitors to Google would hamper the US's ability to surveil and gather data globally.
What is exactly the threat of YouTube becoming an independent company again, e.g.?
The only thing I see is a loss for Google in revenue and in YouTube in the "quality" of their ads.
None of this has any implication on national security.