Ask HN: Why is there a big number of employees in small companies

As a freelance web developer i've worked with a lot of entrepreneurs and made their vision come true by creating their product. Some were simple but others weren't. But i always firmly believed that i could keep most of them floating in my own if it were my company. But sometimes i'd read an article about a small web app that has 50 employees behind it, and just wander to myself "but why though?". A developer to handle the coding, a designer to handle the UI/UX an SEO and marketing expert are more than enough to run a web application business until it grow beyond 10.000 users, in which case each of those 3 could hire four under him to keep the ship flowing etc. I know that i'm probably wrong since almost all web related companies have a big number of employees, at least more than i deem enough. So can anyone explain to me why a company would need that much working force?