This isn't true at all.

The vast vast majority of homes in the US are owned by single family homeowners. And even if that wasn't true it wouldn't make much of a difference. Investment firms don't purchase homes to let them sit there, they purchase homes to rent them out. To whatever degree they're competing against single family homeowners they're just causing a small percentage of housing to shift from owner occupied to rental housing which should actually lower the cost of rentals.

Especially in NYC and SF the cause is very simple: local government has made it illegal to build enough housing to keep pace with demand.