Because when they fake using the Apple trademark for Made-For-iPhone, Apple will get them removed from various market places, retailers, or possibly even stopped at customs when importing them.
Apple has no such enforcement capabilities for USB as they don't have any rights to that trademark.
And we all know how well that works based on how few counterfeits there are on Amazon. /s
When Amazon wasn’t responsive enough they pulled their products and sued suppliers using fulfilled by amazon. USB-if by comparison does nothing and neither do govt agencies- apple is perhaps most active in its enforcement efforts