Because it's not true. People only think this because of an innumerate article from ProPublica.
Almost all landlords are small time, only own one or two buildings, and can't organize a cartel. Except there's one way they can - by changing the law to favor them by banning new construction.
They don't need to collude explicitly. They just watch posted prices in their region and match that. Since the posted prices are always above average (you start higher than the old rent and keep lowering it until somebody takes it), rent keeps going up for everyone.
Rents don't always go up even nominally; you've just listed the upward pressures without the downwards ones. I believe they're still down in SF compared to last year.
Posted prices can be misleading because they prefer to give discounts (X months free) rather than lower the sticker price, so it also depends how you count.
I don't get why there isn't some level of Trust Busting going on regarding the rental property pricing management at all.