Extremely oversimplified:

-for Public Companies: CEO and executives are obliged to pursue maximum profit (either short-term or long-term) for the shareholders, thus the company must grow. - - For shareholders its Cost of capital (basically shareholders want bigger returns than the investment they made) and Opportunity Cost (lose money because you don’t move your investment to a company that is more profitable or gonna be more profitable)

-for private companies: Competition (grow or die from your competitor), efficiency (reducing cost), exit (sell it big and retire), psychological reasons (better safe than sorry), etc…

There are many family business or small companies that function as you describe, but they get replaced and driven out of business in a matter of years or decades (with exceptions). But being stable in an growing economy is very hard and risky. And Capitalism by definition must grow or it gets in crisis.