"I liked Jimi Hendrix's record of this and ever since he died I've being doing it that way..."
- Bob Dylan, on Jimi Hendrix's cover of All Along the Watchtower
Most people are very lazy. They don't want to take the time to think through new ideas or look at them in a new light. Once they've made up their minds about something, they don't change them. That's generally why most people don't come up with ideas for new or great things.
This is also true for many venture capitalists. In fact, it's at the root of a very common question that founders get asked: "Well, isn't so and so doing this?" To be fair, this question isn't necessarily sparked by laziness. It's also sparked by ego - the VC wants to show how familiar they are with the market. They say "Look! I know about things and there's someone else who had the same idea you had." The implicit criticism here is that, because someone else had the idea first, your idea is somehow worse.
I think part of the reason that people ask this question as a way of putting founders down is that they assume that startups are zero sum. That's an assumption born in certain models of markets, but it's completely wrong when looking at startups. Because startups create new value, the idea that someone else has done or is doing something similar to what you're doing often acts to broaden or prove the market you're attacking.
That's not to say that directly cloning another company is a great idea. If you have no differentiation and no original thinking on a problem, then you have to fall to one of two arguments: a) the market for a given idea is so large that there's room for multiple players executing well or b) the other company is so bad at executing that they'll self destruct. They're both potentially valid, but they're hard cases to make - especially at the early stages of a company.
Even though the question might seem dumb, it's one of my favorites. It's also a great question to get as a founder. I ask it of almost every founder I meet, because it's very rare to find a truly new idea. The answer I'm looking for is nearly always "of course someone else has tried this before." But that's not enough. The question begs for a deeper answer, one that talks about why, even though other people have tried the same idea, they're still leaving billions of dollars on the table. It's an opportunity to demonstrate depth of thought and originality. It's that framework of thinking and level of insight that makes greatness.