Why would they bother expelling so much energy to climb so high? Soaring thermals associated with ground heating wouldn't get them that high. They'd have to make the effort themselves.
My guess is that they enjoy the TAS advantage and resultant efficiency that comes with altitude -- same physics as with airplanes. Winds at altitude tend to be faster and they do generally wait for winds in the right direction.
Too bad it's geese and swans that like to fly high. Wing loading likely keeps ducks and loons lower down -- not that you'd want to hit one of those either.
Hit something at 2am. at 8000' -- the landing light gave me enough warning to stuff my head under the panel. Very big bang and messy windshield.