It also assumes free transactions. If I have a cheap $10 per transaction fee, it will hack off 50% of a weekly purchase, or 13% of a monthly purchase. Spacing out much further loses the benefits of dollar cost averaging.
This is probably gets closer to Fast Eddie B's question. For most people, it's not easy to get up and running with a continuous investment habit that pays off in the real world. Their poor credit means a broker won't touch them, or the transaction fees are proportionally high compared to the investment capital, not to mention they probably self-insure or have high deductions, so all their stocks get liquidated to pay for medical crisis funding every few years.
I think I read that almost a third of Americans are 2 weeks away from a financial crisis. Paycheque to paycheque is not a credible investment environment.
Additionally...
Though this has changed relatively recently, it takes money to set up a brokerage account. 35 Years ago, when I started not living from hand to mouth, and could set aside a few dollars a month, I set out to find out how to buy stocks.
I discovered you can't buy stock from a company you want to invest in. You had to buy blocks of stock, minimally, but rarely, 100 shares. Of course, the per share price was more than I was looking at buying for a month. The ones that you could get 100 shares from had the highest per share prices. This is when I learned about Brokerage accounts.
To get one of those, it would take a minimum of $10,000 balance. So, that was right out. But, I learned in places like Money magazine that you didn't have to use bad, expensive Brokerages, it turns out your local bank likely also did Brokerage accounts, and you could do it with a much smaller balance! Yep, if you had a balance in any of your accounts of $100,000, you could set up a brokerage account with a mere $1,000 balance. $2,500 if you didn't put that money in your pillow into their bank.
But, if you don't have the kind of money above, you can still be a "player" in the stock market by buying mutual funds. You buy the mutual fund, they buy the stock. And, the price of entry was only $750.
Fine, I could save my money in a passbook savings account until I saved $2,500. But, I kept hearing I did not need to settle for a passbook savings account. "T-Bills are for you!" the banks told me... if I had $10,000. Don't have $10,000, CDs are for you! No, they weren't.
Hey, Lottery tickets are exciting! It's better than [literally] nothing!