The best explanation is usually the simplest one. But Wall Street and the financial-services industry never got the memo. They always seem to concoct the most complex “solutions” and unwieldy theories of why things happen when the simple truth is right in front of their noses.

Take saving and investing for retirement. For decades, financial advisers and so-called “thought leaders” have wrung their hands and wagged their fingers over Americans’ inability to put away enough to maintain a comfortable lifestyle during retirement. (The median account balance of people who are saving for retirement is less than $60,000, according to the Federal Reserve. But 45% of working households have no retirement savings whatsoever.)

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They have blamed it on people’s ignorance, laziness, or profligacy. They supported the Pension Protection Act of 2006, which allowed employers automatically to enroll workers in 401k retirement plans and put their money into diversified target retirement plans (including stock investments) as a default option.

And yet, Bankrate’s Money Pulse survey, released last week, showed those efforts — and the many articles written by those of us in the financial media — have largely come to naught.

More than half of the 1,001 Americans polled for Bankrate by Princeton Survey Research Associates International don’t own stock at all. That means they’ve missed not only the current bull market but also any chance for long-term market gains, which are key to future retirement security.

And a stunning three out of four millennials, who presumably could afford taking more risk, because they have more time to recover from losses, have no skin in the stock market game.

“ What’s keeping Americans out of the market? Follow the money. ”

Why not? Well, one in five Americans who aren’t invested plead ignorance — they “don’t know enough” about markets. Nearly one in 10 don’t trust the people who would supposedly enlighten them — stock brokers and financial advisers. Another 7% think stocks are too risky, a small number given the two major market crashes that hit investors within a decade.

But the biggest reason people haven’t invested in stocks?

Some 53% of those who don’t own stocks say it’s because they can’t afford to.

This is the big truth staring the financial services industry in the face. All their sales pitches are falling on the deaf ears of a public that’s just getting by.

So, are these people making excuses, as MarketWatch columnist Chuck Jaffe suggested this week? Undoubtedly, many are.

But last year I did the math and found how tough it is to save. If you take the median income of $51,900 and subtract the necessities (food, housing, transportation, taxes, etc.), the average family doesn’t have much left over. That’s why nearly two-thirds of Americans get more than half of their retirement income from Social Security.

In fact, another Bankrate survey in January found that just 38% of Americans “have enough money in their savings accounts to pay for unexpected expenses such as a $1,000 emergency room visit or a $500 car repair.”

The lack of money to invest also explains why few people do other things financial advisers recommend:

Fewer than 3% of retirees wait until they’re 70 years old to get the maximum Social Security benefit, as many suggest. In fact, close to three out of every four Americans start taking Social Security before the full retirement age of 66 or 67.

Less than 3% of American households have 529 college savings plans, even though investment returns in those plans aren’t taxed and states offer additional tax benefits. Those who own 529s have 25 times the wealth of those who don’t.

Roth IRAs, a favorite of planners and TV personal-finance hosts, are increasingly popular with millennials. And yet despite the hype, they accounted for just $500 billion in assets at year-end 2013, a mere 8% of all the money in IRAs, according to the Investment Company Institute, an industry trade group.

The real problem here is not sloth or inertia but income stagnation — yet another casualty of the tectonic shifts in the economy that have produced a widening gap between the affluent, who can afford to save and invest for retirement, and everyone else, who just can’t.

Howard R. Gold is a MarketWatch columnist and founder and editor of GoldenEgg Investing, which offers free market commentary and simple, low-cost, low-risk retirement investing plans. Follow him on Twitter @howardrgold.