Why can’t I, in theory, start a business, grow it until it’s successful, then just…be content and stop growing? Why do people talk about it as inherent to the system to grow like cancer? What about capitalism causes that and why can’t we avoid it? Like, technically speaking?

There are companies out there like that. They typically issue their profits to shareholders as dividends. Others (famously Berkshire Hathaway) will tend to just stack cash waiting on deals for things to buy.