The almost singular answer seems to be employer-sponsored retirement plans - 80% of the 56 million households who own mutual funds buy them through these plans, according to the Investment Company Institute.
-- Nick Ravo, WSJ
The mid-size Silicon Valley company where I work, has its 401K at Fidelity. Normally, I do not keep any money in the "401K" option, since Fidelity has an investment option called Brokeragelink, where you can buy and sell stocks and ETFs, just like a regular brokerage account. Recently I transferred some money from Brokeragelink, back into the regular 401K account, and I had to choose from 30-some mutual funds. Wow! There was only three funds with "ETF-like" expense ratios of less than 0.10%. Everything else, nearly 30 mutual funds, had expense ratios closer to 0.90%.