The key difference between investment and consumption spending is that much of consumption spending is obligatory, while investment is discretionary.
Imagine a middle-class suburban family. Their consumption spending might include a mortgage, a car payment, gasoline, groceries, insurance, medical care, internet, and cell phones. In times of scarcity, they might be able to reduce the costs of some of these items, but most are fixed and unavoidable. The same family, though, in times of high inflation or financial stress, might reduce contributions to pension or savings plans in order to be able to afford basic necessities.
Investment spending is dominated by larger entities, such as businesses, pension funds, and mutual funds, and is more sensitive to things like interest rates and other large-scale economic factors. Investors can choose to hold cash rather than invest it, at time of low interest rates.