I think that a lot of people are forgetting the simple fact that SC2 in addition to being a game is also a simulation of the strategies and tactics used in that era.
While the system adequately simulates blitzkrieg warfare, supply, strategic airpower, and a host of other mechanics, the amphibious rules not only do not simulate reality, but actually go against every established practice and dictum of military strategy.
It is simply impossible to effect a large-scale amphibious assault in the face of superior naval and air power. It's not problematic, or difficult - it's simply impossible. No commander would greenlight such an endeavor to begin with. But SC2 allows you to embark out of range, sail past the inactive and still ships under a planeless sky, and debark on a hostile shore without fear. You could be facing two British air fleets, a strat bomber, and an almost solid wall of ships, but as long as there is just a one-space gap in the line, you can get everyone into the fight.
Like someone pointed out, it takes time to get off the ship. You don't take the landing craft directly. You take a troop ship, and then deploy into the landing craft near the shore. You have tons of support ships, supply ships, troop ships, large and small landing craft, and they're all milling about, loading, getting into formation, being readied, etc. And all during this time the enemy is free to bomb and strafe and shell and torpedo the whole kit and kaboodle.
The whole naval movement within range of the enemy airforce never seemed right to me, like how you can sail German surface ships through the English Channel with impunity as long as you begin and end outside the range of airpower. But surface ships don't break the game. It might be annoying but it's not fatal. But amphibious invasions using that same mechanic is just so totally wrong it's not funny. I can just imagine Eisenhower saying oh the Luftwaffe, they're not a problem. We figured out they can't hit our ships.