Keeping them dependent upon you guarantees that they never surpass you and that you can milk the relationship for all its worth economically and geopolitically. When they have their own fabs spun up it's possible that you are the one eventually stuck with inferior tech, and you lose all sort of revenue + leverage you'd otherwise have from the relationship.

Except it's not a guarantee. They can still surpass you. It might be a little harder, a little less likely - but it's still completely possible.