The simple answer is that "whatever metrics you prefer to use" include qualifications that overseas engineers often cannot satisfy.

Great engineers are human members of a team, and there's more to a team member than their LeetCode score (seriously?) and/or how many GitHub stars they've obtained.

Just some examples off the top of my head:

If you're building products to target US-based consumers, you don't need to provide as much contextual information to a US-based engineer. They will have a stronger intuitive understanding of the product, and there's a better chance that they'll know when it's appropriate to push back and question something. There's a reduced cognitive load in communicating with them. And of course, there's a better chance you're going to enjoy spending hours working with them.