That's a fair point. The key, I think, is understanding when a market is susceptible to entry by another player. But part of making that judgment is understanding the different dimensions along which you can compete, which is why I find The Discipline of Market Leaders so valuable. That was the first book I read that really made the case that it isn't necessarily just about your product. Likewise Crossing The Chasm makes a pretty good case that you can always (well, almost always) segment a market to a degree that there's a place where you can compete.
I’m sure there’s something in their diverse backgrounds that intersect in a way that solves a critical problem. For me, the thing that’s missing in their process is actually building a prototype and getting in front of a user to validate what they think they know.
When you stumble across an idea that's actually viable, it feels like "Oh, why has nobody else done this yet? I better get it out there before the competition eats my lunch." It's very likely that you're actually overestimating the threat of competition, but that's because you have domain knowledge that makes the key differentiators of your product obvious to you but inconceivable to other entrepreneurs.
From your perspective, the idea was obvious and different, and that's why people use your product. From an outside perspective, you entered a crowded market and won it by being better than everybody else. The difference is in the level of detail with which you know the market. If from your perspective it appears to be a crowded market and you just need to be better than everybody else, you probably aren't going to be able to build that better product.