After all, why is Mahadev placed as Shivling? Know its meaning in 3 minutes of rare video

During the excavation of the Indus Valley civilization, the earthly slide was found in many places, proving that Shivling was worshiped from 3500 BC to 2300 BC. A column was praised in the Richas of the Atharvaveda, called Anadi and Anant Pillar, and it is also said to be Brahma himself. From all this, it is understandable that the existence of Shivalinga and the history of worship is very old, but the question is that when the tangible form of Lord Shiva is also present, why have people worshiped in the form of Lord Shiva since ancient times?