They weren't superheroes yet (they worked in the shadows, not publicly), so they didn't have their iconic costumes yet. It was an origin story. In the New 52, Superman started his career wearing a T-shirt and jeans at the age of about 22; the arc ended with him getting his armor. Clark on Smallville started wearing proto-costumes around the age 21, and they evolved until he put on his iconic costume at age 24. That makes his costume wearing start at about the same age as New 52 Superman and earlier than Christopher Reeve's and Henry Cavill's versions of the character. They all donned the costume when they debuted publicly but, like Clark on Man of Steel, Clark on Smallville acted heroically by helping strangers before he wore his suit. In short, Smallville's Clark looked the part when he acted the part (i.e. when he was a public superhero).