Well yes and no. If they are open to the rest of the world a lot of money would end up in Silicon Valley.

If Facebook and Google ran ads all over China, a nice big cut of that revenue goes to the IRS instead of China's government.

The money would only flow there if the services Silicon Valley is producing is worth the money to the purchasers.

None of these transactions are zero-sum and tech isn't the only thing in the world that matters.

Silicon Valley is infamous at this point for running loss leaders to lock people into their ecosystem with the expectation of being able to milk people dry later. The fact that the services are "worth the money" doesn't mean it's economically in China's interests to allow for easy access to them.

Well yes but China has its own replacements that are worth even more to the purchasers because they are highly localized, to a degree that a multi-language global product would never be able to accomplish.

Given that China already has its own equivalent of Facebook and Google, possibly backed by the government. Penetrating that market may prove to be a significant challenge.

That's the parent comment's point: closing the doors led to them being able to build and establish that in-house stuff instead of sending money to other countries...