Both the cart and the karve are unlocked at the same time, and the karve is so much more useful it is ridiculous.
1)First off, the karve has no need for stamina. I can't tell you how dumb and annoying it is to be sprinting up and around hills while using the hoe to make efficient roads. You will frequently gas out, and you will not travel very far at all. Not to mention some of the combat you may end up in. Its no fun running up on a troll or a pack of skeletons when you're completely gassed out.
2) Its not as fast as the Karve. According to the wiki, a maxed out player with 100 sprint goes roughly 13.5 km/h. But this is early game, so you're probably not going to be there, you will be slower. The carve at its slowest speed, which is rowing, meaning you need absolutely no wind to use it, goes 11 km/hour. Thats probably equal, if not slightly faster than a new character pulling a cart. Even if you catch a slight bit of wind with a full sail, you'll be doing double the speed easily, and thats not even in the best boat in the game.
3) You basically have to deal with no enemies when on the boat. If you sail along the coast during the day, there is basically no chance the sea serpent will spawn and attack your ship. And even if it does, you can easily outrun him in the karve. As previously mentioned, in land you have all the mobs to deal with, and odds are you're going to be out of stamina from sprinting and hoeing it up.
4) There is nothing to get hung up on at sea. Every once and a while you might hit a rock, or a bit of land, but for the most part, you won't get stuck on anything. I can't tell you how many shrubs, rocks, and sticks I've gotten my cart stuck on. Its annoying to waste stamina chopping/mining them.
5) You will not roll a boat and have to push it back over. Any time this happens with a cart I think I might have an aneurism.
6) There are so many bits of tin/copper and crypts near the coasts its not even funny. You can pretty much disregard all inland metal sources, just stick to the coasts, and you'll be more than fine.
7) You might think that the storage capacity on the cart has the karve beat, but the struggles above the cart listed makes it no where near as efficient. I can almost guarantee that even with the 4 slots the karve has, I could probably get more copper back to a base faster than you can with the 18 slots the cart has. If we get a longship involved with 18 slots, its not even funny how fast it is.
8) The only obvious glaring area where the cart has the karve beat is on the mountains. Even then, its damn near useless. The hoe isn't quite efficient there, so making pathways sucks. The only thing its good for on the mountain, is pushing off the edge of a cliff for easy collection at the bottom.
I really want to like the cart, but its just so god damn useless compared to the boat.