If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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They don't always. Even if they do, investment doesn't help if it doesn't have anywhere to go. Right now the economy is not investment-limited, it's demand-limited (as you can tell by e.g. low returns on investment).
Why doesn't the market for capital equalize the discrepancy between supply and demand by adjusting the price (the interest rate)?