My understanding is that there's a lot of foreign capital getting parked in real estate in Toronto. I believe they were going to implement a vacancy tax similar to Vancouver next year to try to curb that
No doubt, there's also a great deal of greed and wealth signaling from the general population (especially among upper middle class) as well. Competition around high ranking school areas comes to mind. Not to mention rental properties are pretty much the go-to choice of passive income building that most people go for.
I would bet that if multiple property ownership was penalized by high taxes, real estate prices would come down real fast.