When a company is purchased for amounts above their asset value, goodwill is assumed because there has to be a significant reason some entity is willing to spend more than the asset values to acquire a company.
If you were to allow a company to create their own goodwill, this opens up the door to fraud. Goodwill can only come out of an arms-length transaction, or companies would magically book goodwill entries that could dramatically increase the amount of assets on their balance sheet without any real economic change.

