Investors that gobbled up properties are renting them to renters (assuming it isn't on airbnb) to pay their holding costs (mortgage, tax, maintenance), since they are by definition investors. They actually increase the pool of property for renters, keeping rent price lower, which fights rental inflation. This does drive up purchase price for the home itself, but that's a separate topic.
Same with people parking money, why would they be leaving the homes empty when they could rent it out for additional cash flow?