There are many answers to this.

First, this is not a general capitalism thing. It is more the specific flavor we have.

The original idea of capital investment is that when you need investment for your company (e.g. to buy better machines, expand production, etc.) you let people invest (by buying shares) and then give them a portion of the profits gained from that investment (in the form of dividends).

However, most companies have figured out that if they don’t pay dividends but re-invest the money, shareholders are still happy because their shares get more valuable as the company grows and they get to grow the company, which is good for CEO paychecks.

There are also things like economies of scale (if you produce million units of something per year, it is almost always cheaper per unit than if you produce ten per year). So if you don’t grow, your competitor that does grow could sell cheaper than you and put you out of business.