I'm not the person you're replying to, but one strong reason is that the global reach and standardization of copyright law is far broader than the global reach and standardization of the fair use exception. A single non-US country in which GitHub Copilot is used in a way that would be infringing without the US fair use exception, and outside the scope of any such exception in that law, would be enough to cause GitHub/MS a legal hassle. There could well be more than one such country.
Fair, but GitHub/MS (same company now) can't afford to ignore other countries' law in their internal evaluations of whether globally* available products like Copilot are legal.
* Minus a few countries/regions targeted by US sanctions, I assume, though they've gradually broadened their services in sanctioned countries with the necessary licenses from OFAC.
