When you join a company early, and there aren't many other people employed, you make much more of a difference than if you're one of thousands. You have much more influence in defining the product, steering the direction of the company, defining the workplace culture.

You're not wrong that employees are replaceable, and the company will replace you if it needs or wants to, but in the beginning, it can very much feel like you're part of a group of people who are working together to effectively build lives, support their families, interests, etc. The company isn't just a legal entity that exchanges money for labor, it's a thing you're helping build with a kind of community, that you're investing time into in exchange for the means to a better life.