I'm sure there are a couple more important reasons, but this is the one that I'm thinking about now, so #3 it is.
Outside-in thinking. AKA thinking outside the box. Yes, that one, the one that's fast becoming an old saw, hammer or what-have-you, and is a foundation for many high-priced classes teaching organizations to innovate (does anyone still not know how to connect the dots?). Though tired, it is tried and true and most of us believe that's exactly what we'll be doing.
But there's the inevitable knowing-doing gap. Thinking outside the box can't be mandated, and you can't always rely on peppy (Dr. Pepper-stoked?) facilitators leading your team through mind-expanding exercises like associating recruiting to horses (honestly, I did one of those!). To be truly effective you need creative thought not just during dedicated brain-storming sessions, but also when dealing with to day-to-day demands, be they figuring out the right wording for an email to an angel investor or designing a stop-gap solution to a user problem. That's how a startup gets to be excellent, and not just another venture.
What helps, a lot, is an environment that is conducive to unconstrained thinking. One with constant exposure to fresh thoughts. To people who don't think just like you - at least not all the time (watching Top Chef, ok, but Top Model, huh, why?). And having a marked absence of silos is a big factore. A silo, even if one could be established, would surely crumble in a startup due to the small number of heads and the vast quantity of hats to be filled. Not having a silo means you are not isolated with others who are in the exact same function/department, so you are automatically exposes you to other perspectives. Wow, if the finance guy can routinely listen to the product marketing and the development teams (of one each!) haggling over features, just maybe, every now and then, he has an idea that blows the problem away. Often, it doesn't even have to be an idea, just a question, asked from a different point of view, is all it takes to spark a creative solution.
It is not at all far-fetched to think that the architect's casual idea can spark something for marketing or a tester's question changes the user interface. But many assume that technologists can't get inspiration from other functions like marketing because they're, well, non-technical and can't help solve tech issues. Not so. What the outside-in thinking is all about is to get you out of your current rut and look at the big picture, the forest, the planet even - and that, instead of yet another tech deep-dive, may be what you need to see the solution,. If nothing else, it may re-frame the problem and it may not be such a big deal anymore.
Lastly, it's not just that startups are able to solve problems quicker - the solutions are often more creative than they would be otherwise. as they are informed by the diverse experiences of the entire team. Being small is both a bane and a bounty - startups never seem to have enough staff to get everything done, but they sure rock at innovation.