Because supply is low and demand is high. This is generally the case everywhere, but it's particularly true in large cities and the costal regions of the US. Not enough rentals, lots of tenants looking to rent. That means it's a seller's market (or, rather, a landlord's market). Someone, somewhere will pay what they're charging, so why wouldn't they charge it? It's a business, after all. People rent out their property to make money.
As for why demand is so high, fewer people are buying homes and more people are renting. This breaks down into either people not being able to afford to buy, or people simply choosing to rent out of convenience. As a property manager, I see both situations daily. There's no shortage of renters, even at properties priced well over market rent. Supply and demand. Simple as that.