How can deepwood motte be an old castle while being made of wood? why did the glovers never made an actual fortified structure?

Old castles can be made of wood, you just have to keep repairing them.

Wooden structures are actual fortifications, just as much as stone ones. My guess is that, given the location of Deepwood Motte, wood is a plentiful construction material, whereas stone would have to be imported over significant distances through an oldgrowth forest, making it prohibitively expensive for a lesser House like the Glovers.