1. As PG points out, startups are sorta pass or fail. If you could raise $1m for 25% of your company, or $8m for 25% of your company (which, btw, is a realistic possibility when comparing angel vs VC investors) you would take the $8m because your business has much less chance of going broke. Clearly I'm simplifying here, as there are a lot of other relevant factors (who the investors are, terms, the needed valuation for an exit afterward, etc.) but you get the point.
2. They probably have a lot of ambition. In the case of Flipboard, my guess is they're not thinking "we're just making an iPad app." They're probably setting out to change the way people interact with social media, and the iPad app is just the first step along a very long road.
3. Peace of mind. As an entrepreneur I can tell you, there's something very nice about knowing you're not going to run out of funding for awhile. Everyone breathes easier, and it lets you take bigger gambles when not every product has to be a winner. If you're already wealthy from a previous startup, and you're shooting for a massive exit, your path will involve hiring a lot of people and possibly a large marketing budget. To not have to worry about money requires raising a lot. (This is one of the primary advantages, for first timers, to raising angel money. They don't need an IPO increase their net worth by a factor of 100.)
4. It's what worked for them before. The companies that merged to become PayPal presumably raised a lot along the way. I know TellMe had one $47m round. When people raise a lot of VC money, then have an enormous exit, they're probably going to tend to do the same again. It's what they know, and they have empirical evidence that it can work.
5. Because they can. That shouldn't be a reason, but it definitely is. A big VC round is the startup equivalent of a trophy wife.