Sea trade routes are significantly easier and cheaper to achieve than land based ones of that kind of distance. Additionally at the start of the age of exploration in the 15th century the Duchy of Moscow (as it would be until the mid 16th century) was yet to conquer the majority of Siberia, this would occur though out the 16th and 17th centuries. Before this the land was controlled by various Tartar polities and native Siberians both of whom weren't unknown for banditry and neither of which produced the extensive system of infrastructure required to produce a trade route of the capacity and length required. Even then the Yuan and Manchurian regions of northern China were always a wild frontier to the Ming Dynasty, sometimes suppressed but never truly controlled.

Consider also that while I've been talking about a route to China that's hardly the only trade source that attracted European merchants, you cannot easily access much of the east indies with their lucrative spice trade via Russia.