They spam them because they run out of fuel. AI gains manpower faster than a human player, e.g. expert AI has manpower rate doubled. Once they have no fuel left and their fuel points are captured, they will spam units which need no fuel. AT-spam is probably the most annoying since late game has a lot of tanks and vehicles. Against infantry they can spam StuHs.
AI cuts the wire but not sandbags. It is possible to "cut" sandbags with pioneers/engies but Relic forgot to include that for AI too. Instead they try to destroy sandbags with flamethrowers, grenades and satchel charges. Still you can create kill zones with sandbags and tank traps because they won't destroy them unless they block the way to your troops/base.