Introduction:
Before the 17th century, the existence of cells was not known to scientists. The lack of technology and appropriate instruments prevented their observation. This answer will delve into the reasons why cells could not be observed before the 17th century.

Limited understanding of microscopy:
Microscopy, which is the use of microscopes to view objects that are too small to be seen with the naked eye, was not well understood before the 17th century. Scientists did not possess the knowledge or technology to develop powerful microscopes that could magnify objects to the necessary extent.

Primitiveness of early microscopes:
The early microscopes, such as the single-lens magnifying glass or the compound microscope, had limitations in terms of magnification and resolving power. These primitive instruments were unable to provide clear and detailed images of tiny structures like cells.

Lack of staining techniques:
Staining techniques, which allow the visualization of cells by enhancing their contrast, were not yet developed. Without staining, cells appear transparent and are difficult to distinguish from their surroundings. The absence of staining techniques hindered the observation and identification of cells.

Unawareness of cell existence:
Before the 17th century, scientists were unaware of the concept of cells. The idea that living organisms are composed of smaller structural units called cells was not known. The absence of this knowledge prevented scientists from actively seeking to observe and study cells.

Nonexistent knowledge of cell structure:
The detailed structure of cells, including the presence of organelles and their functions, was not understood prior to the 17th century. Without this knowledge, scientists did not have a clear idea of what to look for when observing microscopic organisms.

Limited availability of specimens:
Access to suitable specimens for microscopic observation was limited before the 17th century. Scientists lacked a wide range of preserved specimens that could be studied under a microscope. This restricted their ability to make significant observations and discoveries.

Conclusion:
In conclusion, the inability to observe cells before the 17th century can be attributed to limited understanding of microscopy, primitiveness of early microscopes, lack of staining techniques, unawareness of cell existence, nonexistent knowledge of cell structure, and limited availability of specimens. It was only in the 17th century with the advancements in microscopy and the discovery of cells by scientists like Robert Hooke that the existence and structure of cells began to be understood.

As cells are small in size there was no instrument to observe them so they were couldn't be seen before 17 century