Why is Land not considered as a fixed capital?
The distinction between land and physical capital is based on some of their economic properties, particularly relating to investment: whether it can be reproduced or not; whether it has a finite lifetime (compared to the human lifetime).

Almost all land is not reproducible, and does not have a finite lifetime.

Physical capital is reproducible, and does have a finite lifetime.

Hence, almost all land is not physical capital.

There are corner cases, where land is reproducible, and may appear and then disappear again in less than a handful of human lifetimes: in those very rare cases, that particular land behaves more like physical capital rather than how land generally behaves, and so could be classed as such.