So there are at least 2.5 arguments for why stock returns are likely to be lower in the next decade or two.
0.5: Lower inflation. You're quoting nominal stock market returns, which include both real changes in price and responses to inflation. From 1946 to 2016 (70 years), the CAGR of the stock market in nominal terms with dividends reinvested was 10.8%. After adjusting for inflation it drops to 6.8%, so 4% of the growth was just inflation and didn't actually influence purchasing power.* However, many commentators assume the fed cannot let the inflation rate get much over 2% going forward, so instead of inflation adding 4% to the long run real CAGR of the stock market, it only adds 2%.
1. Lower P/E ratios. P/E and Shiller P/E ratios are at the relatively high end of historical distributions. How high depends on a bunch of arguments about whether earnings data from between the 1990s is still comparable to earnings data today (after a lot of accounting rules changed). But putting that aside. At a PE ratio of 20, the company is earning 5% of its market cap each year in profits. At a PE ratio of 25, the same company is only earning 4% of its market cap each year in profits. All things being equal, the second company's share price will rise more slowly than the first going forward.
2. Competition from bonds (or lack thereof). Many commentators will bring up the concept of an "equity risk premium." This is essentially the idea that people demand approximately X% higher return to be invested in stocks rather than bonds. Let's say this premium is 4%.** Historically US government bonds had returns of ~5% (nominal). Adding on a 4% risk premium for stocks puts us as 9%, which is not to different from the long term CAGR of the stock market (again in nominal terms). Today interest rates on 10 year government bonds are ~2% (barely equal to inflation). So applying the same risk premium to stocks puts us at 6% nominal returns going forward (only 4% real returns after adjusting for inflation).
But as AZryan did a great job of summarizing, no one actually knows what the stock market will do going forward. And no one knows what the world will be like in 10 years time (but probably a lot more different than most are expecting). I'm just trying to summarize the types of data that are making people think stock market returns may be lower, not arguing whether or not the data is actually good evidence for lower future returns.
*I intentionally chose this window to illustrate the effects of inflation. Prior to the great depression, inflation wasn't really a thing (and sometimes deflation was), so nominal stock market returns were lower, but real returns were pretty much the same.
**Countless papers have been published arguing whether or not the concept makes sense, or, if it does, what the premium actually is, this is for illustration purposes only.