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Some random thoughts about the questions:
Not everyone who participated in the survey is a regular LW reader; it was open to the whole diaspora.
Not everyone who reads LW regularly is also working on their own rationality. Some people are here for the insight porn; some people simply enjoy being in the community of other smart people.
Not everyone who tries to become more rational is doing it correctly. For example, some people may go for the applause lights, or still compartmentalize in something important.
Now, assuming that you are trying to do the rational thing (but of course you are not perfect at it)... Also, assuming you have high intelligence (LW already selects for it), and you are mostly healthy (just a base-rate assumption)...
There are essentially two ways to become more rich: get a high income, or multiply the existing wealth. The second option is not available for those who don't have any significant "existing wealth". For those who do, I guess investing in the passively managed index funds is the standard LW advice.
Assuming that (feel free to adjust the numbers if they feel wrong) you can comfortably live on $2,000 a month, and you believe that index funds will reach at least 4% yearly increase in long term... all you need is to get $600,000, once, and then you can play the rest of your life in the "easy mode". On the other hand, if you start from zero, and you are able to save less than $1,000 a month, the bad news is that you are never going to get there. And if you want to get there in, say, 20 years, you better save about $3,000 a month.
So I guess the answer is that even for smart people, saving $3,000 a month is a difficult task, and 20 years is a long time (LW does not even exist for 20 years yet). In other words, yes, it's true that most LW rationalist are not smart enough to make half a million dollars overnight. But after unpacking "so smart" and "rich", it shouldn't surprise many people. Anasûrimbor Kellhus would probably be able to do it in a few weeks or months, but he is a fictional character.
By the way, from the outside "a person with a lifestyle X" and "a person with a lifestyle X, and with a few thousand dollars saved in index funds" will look the same, even if the latter is richer. Converting the former to latter would mean increasing rationality and wealth, and still would be invisible to outsiders. The difference would be that the latter would see some light at the end of the tunnel, but that light could still be a few decades away.
Society seems to have limits on how poor people can get: bankruptcy, unemployment benefits, not being able to borrow insane amounts of money, etc. (Also, there is the natural limit that people who don't have enough resources to survive, die.) Therefore, unlike intelligence which makes the bell curve, wealth makes an assymetric curve.
Looking from the "99% vs 1%" perspective, we could say that most people actually are relatively poor. That the stupid people aren't much poorer than the average, simply because the average already have very little wealth. Being stupid just means you will waste a little more of your money, until you are out of money, and then you can't waste anymore.
You can't become a "negative Bill Gates"; at worst you can become homeless (and then die). Actually, if you are just smart enough to pay your mortgage first, and only do the stupid things with the remaining money, you will probably even avoid homelessness. The average and below-average people have a script to follow, which will more or less keep them in the fixed position, as long as they can have a job.
The LW surveys contain questions about whether people are regular LW readers and allow us to see how people who are regular readers differ.