They don't need to be, as long as the public knows what they represent. Wrestlers and and bodybuilders are associated with great physical strength. They wear trunks. Superman wearing trunks shares the same artistic symbolism. It's about creating impressions in the readers minds, not about looking exactly like something that's trendy.
Also trunks are only part of the imagery. Other features are also needed, such as the cape to give a hint of majesty and to create the impression of dynamic movement in flight, or the chest logo to create the image of sturdiness. Again no-one is saying Superman should dress exactly like a modern wrestler, just that his look should borrow elements of the attire that suit him, in this case the trunks.