Because intelligence != success, nor is it enough to obtain it. You can be in the top 0.01% of the population IQ wise with all As in every subject and a ton of degrees and experience, and not get anywhere close to being a millionaire or billionaire.
What matters a lot more is the combination of a bunch of factors:
1. Your network/background. Someone with a rich and influential family has way better odds than someone from a poorer one.
2. Whether your interests happen to align with a field or industry that's got a lot of potential for success. Being interested in technology/bitcoin/AI/whatever at the right time would get you a lot more money than being interested in journalism or travel.
3. Your mindset and how determined you are to push yourselves to do one thing despite overwhelmingly bad odds. Obviously you're gonna have issues if you get really attached to a bad idea with no chance of it becoming a viable business venture, but I also see a lot of people fail simply because their interests are very temporary/short term.
4. Plain old luck when it comes to your skillset, the timing for launching your idea, etc.
Point being though, there's a huge luck/timing element to it all, with connections and your network being another big factor on top of that. If you don't get those, you can be as skilled and hard working as you like and not get anywhere.