Because they have vastly more incentive to have them made on the island, then anyone else would have on their territory.
And the Taiwanese employees have more external motivation then anyone could feasibly have in the U.S. or Europe.
https://www.reddit.com/r/hardware/comments/wzqtd1/chinese_it...
Apple make iPhone that everyone love. Apple is the Shield that protect USA or maybe Google, Facebook? how about Ukraine is the food basket of Europe or perhaps even the world since Ukraine is a big exporter of grains. surely no one will attack Ukraine because the food price will soar and possible world wide famine.
Some might consider it unlikely that we actually would show up, but they are a small island off the shore of a superpower, so they are playing the hand they were dealt as well as they can. And, a small chance of the US showing up is still a pretty big disincentive to starting an invasion.
China needs to deactivate or sabotage the explosives before any invasion. Even in that case, I am sure there is a plan B to nuke the factories.
Not only that, it's very dependent on the skills and hard work of its staff. Which is one of the things that makes TSMC so hard to replicate.