Correct but also a company like Google knows that the moment they put up a link in the frontpage they can drive vast amounts of traffic towards the new property

Correct but also a company like Google knows that the moment they put up a link in the frontpage they can drive vast amounts of traffic towards the new property. Secondary reason is that a web app from Google that doesn't scale is really bad PR and draws comments like "omg, lulz it's Google and they can't handle traffic???!!!1!" Hence, they build with scaling in mind. A startup has none of these concerns and just cares about time to market and making some money to convince VC's that it's worth investing.