i have to replace the back tires on my kia sportage 2013 constantly why

There is no chamber adjustment on the rear of the Sportage. How often is constantly in miles? What brand tires are you using? Worn suspension components? Are you rotating the tires front to back every 7500 miles?

Drive straight through a puddle and look at the tire tracks behind you. There should be only one set of wet tracks. If not, the vehicle is crabbing. In other words not going down the road straight. Most likely it was in a collision at some point. A vehicle history report such as Carfax will confirm that. If that's what has happened it'll never be right. Hope that helps! Jim