If I recall, a lot of people tried to mod this and ground transports in when the game launched. No one ever got it working.
I don't know why, and of course I love the "d-day" landing cinematic thoughts, but with the game popping in a few engies and building factories under the artillery coverage of your navy on the shore, is how that kind of thing is done here, and that's fine. If the game was more set up so you produced units centrally, then had to get them to the fight, a logistics game like Creeper World 3/4, then such transports make a lot of sense.
I try to make my units centrally a lot, using air transport assists or ferry points to get them from my centralized production to the front lines, and it's tough to get working, and easily shut down by competent enemy air. I'd love more transports. But in the end the game is set up for you to make factories near the front.