2007 wheel bearing hub assembly question

[cedarwoodron]

I have a 2007 Mercury Mountaineer AWD that has been checked when I recently rotated my tires and I was told that the front wheel bearings L&R need replacement. I can hear a low sound in the cabin when I drive most recently, so I don't dispute this, as all other mechanic systems -brakes, steering and suspension were found to be in good shape.
My question is this- why can't just the wheel bearings be replaced in the hub units , instead of having to buy 2 new entire hubs!
Cedarwoodron

 

[MotorCityFats13]

they are sealed bearing hubs you dont have to replace them in pairs but you have to replace the whole thing because its not a serviceable unit .... these are nothing like the old style where you pack bearings in grease and install them onto a spindle