Further, the properties held purely as assets are generally very expensive ones. The effect ripples down, well heeled people move into cheaper housing and those people moving in turn.
This is nearly 8 years old, but generally describes the issue with some specifics:
https://www.theguardian.com/society/2014/jan/31/inside-londo...
Vacancy rates are extremely low. That puts an upper limit on the amount of people that could be doing that. Is it not easy to tell if a home is vacant?
Some very select markets may have an actual problem if foreign money laundering is a thing (as I’ve heard is true of Vancouver).
Lowering rates can cause loan problems.
Renters can choose not to pay rent, can cause untold damage to the property and can use it for illegal purposes to name but a few reasons why it might be rational not to rent a property out, specially if it's an appreciating asset.
I suspect that the majority of these empty properties are owned by wealthy people from countries like China or Russia.