Explain Like I'm 5 (ELI5)

Because, according to capitalism, growth benefits everyone. Both the wealthy and the poor. The rich get richer, the poor get jobs. They can all be taxed and that tax can then be spent on improving public services and infrastructure, which then also benefits the unemployed. The larger your business grows, the more wealth it generates, the more jobs in provides, the more tax it pays. So, by that logic, growth is seen as the best indicator of success.