It's not, though. The classical factors of production, whence we get the concept of "capital" as a factor of production, has land and capital as clearly separate:

Land or natural resource — naturally occurring goods like water, air, soil, minerals, flora, fauna and climate that are used in the creation of products. The payment given to a landowner is rent, loyalties, commission and goodwill.

Labor — human effort used in production which also includes technical and marketing expertise. The payment for someone else's labor and all income received from one's own labor is wages. Labor can also be classified as the physical and mental contribution of an employee to the production of the good(s).

Capital stock — human-made goods which are used in the production of other goods. These include machinery, tools, and buildings. They are of two types, fixed and working. Fixed are one time investments like machines, tools and working consists of liquid cash or money in hand and raw material.

https://en.m.wikipedia.org/wiki/Factors_of_production

And it's an important distinction. The fact that land is not made and inherently finite makes it zero-sum. Meanwhile, the fact that capital such as education, tools, factories, infrastructure, etc. are man-made and not inherently finite makes them not zero-sum. This distinction has truly massive implications when it comes to economics and policymaking. It's the whole reason LVT is so effective, so efficient, and so fair: it exploits the unique zero-sum nature of land.