[oneplane]

Efficiency, specialisation, scale. You doing one thing on your own vs. a collection of many of those 'things' you used to do solo, guess which one has more potential?

This does or course not mean that this applies all the time, everywhere, forever. But in this case, the 'startup' merely has a model of economy of scale, and then probably making sure the benefits are big enough that they can scrape a bit of a provision off of it. That way they can make money while still delivering something that has a higher value to a customer than the customer doing it themselves.