On the other hand, most computer systems employ fewer layers than the OSI model requires. Why not use a smaller number of layers? What are some of the disadvantages of using fewer layers?
Your question is about on the number of OSI layers.
Let's solve your question with explanation.
Your first question is Why not use a smaller number of layers?
The reason behind that are
It does not distinguish between the data link and physical layers. Which has most amount of different functionality.
The data link layer can relate to transmission of the frames. And the physical layers should lay down the physical functionality of transmission.
They does not separate the fundamental of service, protocol and interface.
These are the mostly main reason behind the not use of smaller number of layers.
Your second question is what are the disadvantages of using fewer layers?
Fewer layers are little bit complicated layer the are difficult to manage.
Transport layers doesn't give guarantee to delivery of the package.
It is not easy to replace protocol.
Fewer layers are designed for the wide are network not optimize for local area network or personal area network.
Fewer layers give complexity.
These are the main disadvantages of fewer layers.
These are all that you want. thank you.