The hedgerows in Normandy - the Bocage - were almost like having trenches already set-up in thousands of places across the country. So no need to dig more. Plus the pinned down mechanic, where units can take less losses, can assume that infantrymen immediately get down into any ditches, shell holes etc
You don't have to have a meeting engagement of course, you can setup a defensive line, though you have to occupy it at the start of the game, and it needs to be very close to the centre of the map, or you lose too many points. It's certainly possible on some maps, and then allow your opponent to expend all of his effort in fruitless attacks before counter-attacking late in the game to secure victory.
BTW there is at least one bunker on the Omaha map which your men can occupy, and it's certainly possible for Eugen to stage at least one map in the campaign with a semi-fortified line or for modders to do it. Whether the AI would understand to occupy that particular position is another thing. Or Eugen would be interested in any very asymmetric maps, which force one side to be defender, the other attacker. Which is presumably why there appears to be no beach assault against the Atlantic Wall defenses. The Allies are already landed and are in the process of building up troops to breakout of the beachhead.
Still, in RD there were scenarios in the Busan Pocket campaign, where you did have to form a good defensive line and just hold on for dear life as you got assaulted by millions of North Koreans, or in the Hong Kong campaign facing the Chinese. I can foresee this in a UK scenario where light British forces face a German Panzer counter-attack at Pegasus Bridge. Not a fortified position, but defending from one side of the river/canal. And a German scenario where they have to defend Cherbourg/US scenario where they decide to assault it for the port facilities. On the WG and RD campaign maps, controlling a port allowed you to bring in more units from overseas. The SD campaign should allow the Allies this strategic option, where you can transport over the Channel some of Patton's Divisions quicker, if you have a port.