A lot of them just aren't great ideas to start with. This for example would create a lot of land but much of it would be pretty salty so wouldn't grow food ver

A lot of them just aren't great ideas to start with. This for example would create a lot of land but much of it would be pretty salty so wouldn't grow food very well. In addition the remaining water would slowly get saltier and saltier eventually probably dying similar to the Dead Sea, granted this would take a while but it's a definite negative, the Med is already saltier than most of the ocean because it's relatively isolated from the larger circulation of the Atlantic and Indian oceans with a large surface area where evaporation takes place. Then there are some like the city mega-projects that require a huge amount of social infrastructure that's hard to build, take a look at some of China's mega construction where they've tried to build cities out of whole cloth but the people haven't really come so they're just empty.