Interest rates are too low and with talk of the possibility of negative interest rates there are no incentives to save money. When was the last time you heard of a certificate of deposit (CD) offering by a bank?
Your point may stand in the event of negative interest rates, but generally, it seems like you're assuming that what Americans are doing is finding investing avenues that offer greater returns than prevailing interest rates, which does not appear to be the case. Saving money will, by definition, leave you with more money than spending (not investing) it, even if the ROI is near 0.