The plan, such as it was, was to barrage the defenders with aircraft, naval gunfire support and then rockets and other artillery from landing vessels themselves. That was supposed to suppress the defenders enough to give the lads attacking something of a sporting chance.

/u/the_howling_cow has had a bit of a crack at this question over here. https://www.reddit.com/r/AskHistorians/comments/620zob/why_did_the_american_forces_land_on_normandy_with/dfix9nk/

But, fundamentally, even with jumping out the front ramp, soldiers still had to wade through a bit of water. If they jumped out the side or rear, then they could be landing in water deeper than they could stand in, and drowning is not a pleasant way of arriving in France.