Common reason is comfort but I think the loud colors are purely for show. Just like having a flashy car. They make cycling kits that look like every day wear,

Common reason is comfort but I think the loud colors are purely for show. Just like having a flashy car. They make cycling kits that look like every day wear, especially for mountain biking where having a bit if extra material is extremely helpful when you run out of talent and crash ;). Personally, I just wear cargo shorts with gussets and a brightly colored exercise shirt (like the kind worn for running). Visibility and comfort without looking like a billboard.