For the D-Day landings, why did we have soldiers land in slow unarmed boats then have them try to get off the beaches amidst a hail of bullets? Why couldn't we have them parachute in, which seems to me a lot less risky? I'm sure there's a good reason for it, and I'd like to know.
1. Paratroopers were landed. Rather a lot of them. See Pegasus Bridge, and many others.
2. Paratroopers drop from the air. They are subject to scattering by wind.
3. Planes sometimes get navigation wrong, and drop in the wrong place.
4. Paratroopers can't take heavy equipment with them. They are light infantry without armour, artillery, or pretty much anything above what a guy can carry.
5. Paratroopers will take around 10-25% casualties on landing (depending on wind and terrain) even before the enemy start doing anything. Broken ankles, that sort of stuff.
6. Once they land, they have to walk to wherever they need to get to. No vehicles.
7. They will land scattered. The worse the wind conditions and the higher the drop, the greater the scattering. They will also land in the middle of the enemy. That's potentially a problem.
8. You don't want to think about the mess that a couple of enemy fighters getting into the drop would cause.
9. It takes a long time to teach people how to jump out of an aircraft and survive the experience.
10. Once you step out of a plane, there's no way of getting back if you discover things have gone pear-shaped. Worst case on a beach, you can pull troops off. Worst case on a drop, there's nothing to be done.
11. Paratroopers need a plane to jump out of. The number of paratroopers is limited by the number of planes. Landing boats are quick and easy to build. Planes, not so much.