Because you have seen this scenario before. You invest using the services of a broker that sends you a few generic reports on the state of the market as a whole. Sometimes you even get a perfunctory analysis in one particular company, but not a value-based analysis. You never see much growth in your savings. Why? Because as Warren Buffett said, to make money in the stock market it is imperative to keep the fees of doing business to a minimum. In addition, when things go awry and the market crashes, where is your broker? If you do not mind your own business, no one will.
Fidelity and other financial institutions manage hundreds of ETFs that allow you to trade with no commission as long as you hold them for 5 weeks. If you know what is going on, you can trade once every 5 weeks without paying fees. Not only you'll know what is going on, you'll do it almost for free.
Even though the answer to this question will vary depending who you ask, I have to say that whenever things are going okay - the economy is growing, the yield curve is normal, and the stock market valuation is within normal levels, I would rather lean towards ETFs. When any of the previous conditions are not there, I would start looking into specific companies.
Even though there's no infallible economic measurement to detect market crashes before they happen, there are indeed a collection of measurements you can use to predict if it's about to happen or if you still have time. The dynamic yield curve is one, the relation between the stock market and the GDP is another one, the inflation and unemployment rate are others.