What are the reasons behind the testing model called the pyramid?

The testing model in the form of a pyramid got its name because of its visual structure, resembling a triangle or pyramid. At the bottom is the widest layer, consisting of a large number of unit tests that check individual modules or functions of the code. The middle layer is integration tests, fewer in number, checking the interaction between components. The top, narrowest layer is UI/end-to-end tests, which are the most resource-intensive and slow, so there should be the fewest of them.

This distribution reflects an optimal balance between speed, cost, and reliability of testing: many fast and cheap unit tests, fewer integration tests, and even fewer slow UI tests. Therefore, the pyramid shape symbolizes the quantity and level of tests at each stage.