We’ve all got veins in our legs quite near to the skin surface, but they are hidden by a layer of fat just under the skin.
Tour de France cyclist's legs appear to be uber-veiny for two main reasons: firstly, they have much less body fat than ‘ordinary’ people, and secondly, their veins and arteries have adapted to carry more blood around their bodies. The cardiovascular adaptations are numerous, but a large increase in vein and artery diameter is one of them. You can read more about the science behind why Tour de France rider’s legs are so veiny here.