Why is the dominant investing advice for individuals to use mutual funds, exchanged traded funds (ETFs), etc

Great question! While investing in individual stocks can be very useful as a learning experience, my opinion is that concentrating an entire portfolio in a few companies' stock is a mistake for most investors, and especially for a novice for several reasons. After all, only a handful of professional investors have ever beaten the market over the long term by picking stocks, so is it really worth trying? If you could, I'd say go work on Wall Street and good luck to you.

Diversification

For many investors, diversification is an important reason to use an ETF or index fund. If they were to focus on a few sectors or companies, it is more likely that they would have a lop-sided risk profile and might be subject to a larger downside risk potential than the market as a whole, i.e. "don't put all your eggs in one basket". Diversification is important because of the nature of compound investing -- if you take a significant hit, it will take you a long time to recover because all of your future gains are building off of a smaller base. This is one reason that younger investors often take a larger position in equities, as they have longer to recover from significant market declines. While it is very possible to build a balanced, diversified portfolio from individual stocks, this isn't something I'd recommend for a new investor and would require a substantial college-level understanding of investments, and in any case, this portfolio would have a more discrete efficient frontier than the market as a whole.

Lower Volatility

Picking individual stocks or sectors would could also significantly increase or decrease the overall volatility of the portfolio relative to the market, especially if the stocks are highly cyclical or correlated to the same market factors. So if they are buying tech stocks, they might see bigger upswings and downswings compared to the market as a whole, or see the opposite effect in the case of utilities. In other words, owning a basket of individual stocks may result in an unintended volatility/beta profile.

Lower Trading Costs and Taxes

Investors who buy individual stocks tend to trade more in an attempt to beat the market. After accounting for commission fees, transaction costs (bid/ask spread), and taxes, most individual investors get only a fraction of the market average return. One famous academic study finds that investors who trade more tend to trail the stock market more. Trading also tends to incur higher taxes since short term gains (<1 year) are taxed at marginal income tax rates that are higher than long term capital gains.

Investors tend to trade due to behavioral failures such as trying to time the market, being overconfident, speculating on stocks instead of long-term investing, following what everyone else is doing, and getting in and out of the market as a result of an emotional reaction to volatility (ie buying when stocks are high/rising and selling when they are low/falling). Investing in index funds can involve minimal fees and discourages behavior that causes investors to incur excessive trading costs. This can make a big difference over the long run as extra costs and taxes compound significantly over time.

It's Hard to Beat the Market since Markets are Quite Efficient

Another reason to use funds is that it is reasonable to assume that at any point in time, the market does a fairly good job of pricing securities based on all known information. In other words, if a given stock is trading at a low P/E relative to the market, the market as a whole has decided that there is good reason for this valuation. This idea is based on the assumption that there are already so many professional analysts and traders looking for arbitrage opportunities that few such opportunities exist, and where they do exist, persist for only a short time. If you accept this theory generally (obviously, the market is not perfect), there is very little in the way of insight on pricing that the average novice investor could provide given limited knowledge of the markets and only a few hours of research. It might be more likely that opportunities identified by the novice would reflect omissions of relevant information. Trying to make money in this way then becomes a bet that other informed, professional investors are wrong and you are right (options traders, for example).

Prices are Unpredictable (Behave Like "Random" Walks)

If you want to make money as a long-term investor/owner rather than a speculator/trader, than most of the future change in asset prices will be a result of future events and information that is not yet known. Since no one knows how the world will change or who will be tomorrow's winners or losers, much less in 30 years, this is sometimes referred to as a "random walk." You can point to fundamental analysis and say "X company has great free cash flow, so I will invest in them", but ultimately, the problem with this type of analysis is that everyone else has already done it too. For example, Warren Buffett famously already knows the price at which he'd buy every company he's interested in buying. When everyone else can do the same analysis as you, the price already reflects the market's take on that public information (Efficent Market theory), and what is left is the unknown (I wouldn't use the term "random").

Overall, I think there is simply a very large potential for an individual investor to make a few mistakes with individual stocks over 20+ years that will really cost a lot, and I think most investors want a balance of risk and return versus the largest possible return, and don't have an interest in developing a professional knowledge of stocks. I think a better strategy for most investors is to share in the future profits of companies by holding a well-diversified portfolio for the long term and to avoid making a large number of decisions about which stocks to own.