Why should the future returns of stocks always be the same as their past returns?

As the enduring antidote to this kind of bull-market baloney, Graham
urges the intelligent investor to ask some simple, skeptical questions.
Why should the future returns of stocks always be the same as their past
returns? When every investor comes to believe that stocks are guaranteed
to make money in the long run, won't the market end up being wildly
overpriced? And once that happens, how can future returns possibly be high?

Why should the future returns of stocks always be the same as their past returns?