They're a thing, they just don't get much focus.
I believe Tank Witches might actually be more common in-universe, as land strikers require less magic to use (which allows the user to focus their magic on bigger guns and better shields), but of course they are useless against flying Neuroi.
Land Strikers are basically giant metal legs, the witches can walk around with them or use the deployable wheels to dash.
The series focuses on international Joint Fighter Wings, which are only for aerial Witches, while Tank Witches mostly stick to their respective countries. The girl in the OP was originally a Tank Witch for the Fuso Army. Aurora now leads a support unit for the 502nd, who's main job is to carry Nipa's stuff back whenever she crashes.