old gods do new jobs
If you're confused, you might need a reminder (or explanation) that in this scene in Thor: The Dark World, Loki transforms himself into a likeness of Steve Rogers in the Captain America suit to needle Thor. Tom Hiddleston explains that this scene was filmed and then shown to Chris Evans, so that he could capture Loki as Cap in his own performance, "It's him doing an impression of me doing an impression of him, and it's brilliant." On the contrary, Mr. Hiddleston, it's Chris Evans doing an impression of your impression of Loki's impression of Captain America. We need to go deeper.
But that still doesn't explain why this scene required Tom Hiddleston to wear his own Captain America suit.