Because there was no visible life, no green, no plants, except for perhaps some very limited cyanobacteria. That is the subject we now explore. Land plants probably evolved from marine plants, moved into freshwater and finally onto land. Lack specialized cells for the movement of nutrients and water. They are small and limited mostly to low, moist areas.
Leaves are wide but thin to collect the maximum amount of sunlight at minimal metabolic cost.
Note the wood cell vascular system visible in this well-preserved specimen. Photo by G. Mead. Still most land areas were barren. By some 420 m.y.a. These seedless vascular plants required water for sperm to reach the eggs and were still limited in their distribution to wet climates.