Because a) so many people have abused the privilege of having animals in a rental, and b) landlords have discovered that responsible pet owners who love their pets are willing to pay just about anything to keep them. It's completely insane...so far the cheapest we've been able to find in my area for a 15lb. dog is $600 a year on top of the regular rent, plus an additional $200 deposit on top of the usual security deposit. Considering that we're looking for a long-term rental, by time all is said and done our dog would pay for a complete interior and landscaping makeover on the place.
Alot of the time it's not the pet it's the pet owners that allow the dog to ruin things. Their are ways to curve destructive behavior. I review Dog Toys and their are alot that keep their attention for a while.