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.
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?
But lets assume the worst, that it is all just pure evil capitalist speculation: even that is not necessarily bad. They try to anticipate future demand. So they anticipate that in the (near) future, somebody will need that apartment even more urgently than the people looking for housing right now. So it is better to keep it available for those people in the future. Think about keeping ICU units ready for future Covid patients for an analogy.
So what if the speculators are right, and the future buyers really have a higher need than the current ones?
Also I think if there really is just speculation, especially foreign investors, why not build cheap houses and sell them to the speculators for a lot of money? Nobody will ever live there (according to the anti-capitalists), anyway. Seems like a good way to make money for a country.