Another one: Doing market research well is really hard, and not something the person/group has a lot of experience in. It also might result in finding out that

Another one: Doing market research well is really hard, and not something the person/group has a lot of experience in. It also might result in finding out that there isn't really a market which means we would have to abandon or drastically change our plan, and we really don't want to do that. So, let's just not bother and assume there is a market! Then there's the fun dynamic where if you are being really honest with yourself you don't care too much about whether there is a market or not, you just want to see the idea in your head become reality, because it would be super cool and you will feel good for having been able to do it. Which might be fine for a one-person side project, but otherwise not so much.