This question carries the assumption that life is some sort of race in which we are all struggling to outdo everyone else. Life is not like that. We are not racing against others because we all have different characters and different goals. There is a different finish line for each of us. Even the pace at which we choose to pursue our goals varies with the character of individuals. One widely-held aim of parents is to give their children a decent life, even a better life than they had themselves. Many put effort into achieving this; it seems to be a natural and widespread aim. When society prevents them from passing on their assets to their children after death, they often find other ways of achieving that aim. This can take the form of using influence to place their children in comfortable jobs. It can be done by transferring the assets while they are still alive. It can lead people to set up complex trust schemes beyond the reach of the taxman.
Inheritance Tax is taxing money that has already been taxed when it was earned. The provision parents worked to make for their children, paying tax as they did so, is now taxed again, removing part of their incentive to create wealth in the process. For many recipients, the bequest comes as a lump sum when they are already established and probably own their home. It is thus available for investment or to start a business. Taxing it greatly reduces these possibilities. The capital pools built up by a family business such as a shop, for example, can be dissipated on death by Inheritance Tax, with a consequent economic loss to society, a loss that impacts employees and customers.
People are not equal and cannot be made so. They are differently talented. Some are genetically equipped to develop athletic prowess if they work at it. For others it might be music or mathematics. The notion that 'fairness' requires they should all have equal chances sits ill with what life is actually like. We should not be trying to impose an equality that does not fit, but on extending to everyone the opportunity to better their lives.