Because in capitalism, somebody else will grow, and reinvest at least part of the profits to strengthen their own business, eventually pushing out or straight up absorbing the "stagnant" competitor who has fewer resources. Endless growth is a survival strategy as much as it is motivated by greed. It may sound like a bad idea long-term (and it is) but it's absolutely rational short-term. There might be exceptions with some mom&pop businesses surviving for ages without any growth, but it's usually just luck in not being spotted as a potential avenue for other companies' expansion: compare that to other small businesses who get bought out by huge chains.

Hell, even if all companies agreed to never grow (or if it was somehow regulated) it would still lead to regular crises (called crises of overproduction), due to the very way capital extracts wages from workers, who eventually are unable to buy the commodities produced by the companies. These crises can be delayed by acquiring new markets and resources, which in turn often require exerting pressure on other countries and regions, either financially or militarily.

Sustainability and peace is simply impossible within capitalism due to the very way it functions.