Because, in a nutshell, the "economy" is a massive ponzi scheme that depends on constant growth. This isn't a popular opinion among many of the VC crowd that believe in the myth of the meritocracy, but its an irrefutable truth. For example, the FED (and other central banks) set a target inflation rate (usually around 2%, but it can vary). That means if you are "stable", and the central bank has its way, you are actually shrinking.
Its obviously much more complex then this, but that is the crux of it. Unfortunately "sustainability", whether economic or environmental, isn't a priority for those .000001% of the oligarchs that run the show and make their money off debt servicing.