The question is based on somewhat flawed premises. In fact, the 'Silk Road' in its broadest sense is often taken to include arguably more voluminous oceanic stretches as well, with the best-studied being connections in the western Indian Ocean (see /u/Tiako's post on the routes in Roman times). This post by /u/SandaFluffoid goes into more detail than I can, but it seems that Persians have traditionally also been relatively active seafarers, but what made China exceptional is that South China, where the best locations for trading ports were, was not heavily colonised before agricultural innovations in the ninth century that were able to overcome its generally poor terrain. As such, it wasn't until the Song Dynasty emerged in the late 10th century that there began to be a serious Chinese maritime trade out of Fujian, Quanzhou, Canton and so on. (As an aside, there's a fascinating discussion in Serge Gruzinski's The Eagle and the Dragon of how Columbus believed he had arrived near one of these Chinese ports on first landing in Hispaniola, but that's for another time.) Without those ports, then it was the landward route through Southeast Asia towards India or across the steppes towards Persia that were the most readily available. But, as this post by /u/anchaescastilla notes, the route was always a series of relatively local hops, and rarely if ever did a single merchant seek to travel more than a few hops either direction. While states occasionally sponsored long-range diplomatic missions over land, merchants tended not to stray too far from safety.