The Simpsons were designed as the typical american family in every way. When the writers invented them having a family trust they were making a statement that

The Simpsons were designed as the typical american family in every way. When the writers invented them having a family trust they were making a statement that such trusts are commonplace and normal in American society, at least normal enough that the audience would understand the concept. Thirty years ago very few average people would have even known the word. Things have changes since the series started.