Social Security serves as a critical source of income for millions of retirees today. But most Americans are depending way too heavily on those benefits.

It's estimated that 62% of retirees count on Social Security for at least half of their monthly income, while 34% rely on it for 90% to 100% of their monthly income. Both statistics are frightening, particularly the latter. That's because Social Security was never designed to sustain retirees on its own. Rather, the point of Social Security is to supplement another income source, whether a pension, 401(k), IRA, or ongoing work in retirement. In fact, in a best-case scenario, it'll replace about 40% of the average worker's pre-retirement income, but most seniors need twice (or close to twice) that amount to cover the bills.

Social Security card being held between a senior's fingers


So how did we get to a place where there's such an overreliance on Social Security? Here are a few reasons why seniors land in this predicament and how to avoid them.

1. They don't save enough (or at all) during their working years

Many workers neglect to save for retirement for a variety of reasons. Some don't understand Social Security's limitations. Others have their paychecks monopolized by living expenses. But while it's easy to make excuses for not saving, the fact of the matter is that one-third of baby boomers today have less than $10,000 in a retirement plan, which means they're running out of time to salvage their golden years. Younger workers aren't necessarily much better off, though -- 42% of all Americans have less than $10,000 socked away for the future.

No matter your age, you must make an effort to save during your working years to avoid running out of money later in life. Currently, workers under 50 can contribute up to $18,500 annually to a 401(k), and $5,500 to an IRA. Those 50 and over, meanwhile, can put away up to $24,500 a year in a 401(k), and $6,500 in an IRA.

If you're in your 50s or 60s without much savings, be sure to take advantage of these limits and catch up. Maxing out a 401(k) for just five years, for example, will leave you with an additional $122,500 for retirement, and that's not including investment growth during that time.

If you're younger, you don't necessarily need to max out a 401(k) to build your nest egg (though it's certainly not a bad idea). Contributing $500 a month over 30 years, for example, will give you $567,000 if your investments bring in an average annual 7% return during that time. Just be sure to do something, because if you fail to save a dime, you'll be forced to bank on Social Security to an unhealthy extent.

2. They save well but invest poorly

It's estimated that 60% of Americans invest too conservatively for retirement by focusing on avoiding losses rather than maximizing growth. But that's an approach that could cause you to have no choice but to fall back on Social Security.

In order to keep pace with inflation, you need the money you save today to grow substantially over time. In the example above, we saw that a monthly contribution of $500 could grow to $567,000 over 30 years with a 7% average annual return, which is what you're likely to get with a stock-heavy portfolio. But if you play it safe and keep your money mostly in bonds and cash, you might generate just a 2% return over time, which would leave you with only $243,000 in the aforementioned scenario.

While saving money consistently is a good way to avoid relying too heavily on Social Security, don't derail that effort by botching your investments. If you're in your 30s and 40s, you're generally good to put about 70% to 80% of your nest egg in stocks. As you get older, that percentage should decrease, but the point is to make stocks a reasonable part of your investment strategy, even as you're nearing retirement.

3. They save well, invest wisely, but get clobbered by healthcare costs

If there's one expense that tends to catch seniors off guard, it's healthcare. The average healthy couple today will spend roughly $400,000 or more on medical care in retirement, and that's not even including the cost of long-term care. So even folks with decent nest eggs wind up having to rely on Social Security to maintain their health.

Of course, there's nothing wrong with using Social Security to help cover your senior medical bills, but you're better off saving for those expenses independently during your working years to ensure that you're truly covered. And part of saving appropriately means reading up on what healthcare will cost you as a senior to get a sense of what you're dealing with.

If you worked and paid Social Security taxes throughout your career, then those benefits are something you're absolutely entitled to -- but that doesn't mean you should count on them to an unhealthy degree. Save independently, invest efficiently, and prepare for the glaring expense that is healthcare so you're not utterly walloped once your career comes to a close. It's a better bet than relying too much on Social Security and falling short as a result.