Yeah, this is true of everything from oil to fruit to toasters.
Although I would argue that the price tag of rentals has a strong relationship to what it costs to build/operate additional units of rental space. While one landlord might have had their unit paid for 3x, where would a new entrant to the market be?
At least in Toronto, the answer is, losing money.
https://www.cbc.ca/news/canada/toronto/gta-condo-investors-l...
Same with fruit and oil. When avocado soared in popularity, new people entered the market until there was a price equilibrium.