It's possible the localiser antennae work better off a flat surface than somewhere up in the air. The navaid folks may be able to comment knowledgeably.
I'm not so sure that a berm is cheaper than a gantry as moving that amount of earth and making it stable ain't cheap.
If, for argument's sake, there was a frangible gantry instead of a berm, the prospects for a successful takeoff after going through a gantry would be diminished by the indigestion the engines would suffer as some of the pieces were sucked in.