answer:Because we don’t live in Venice? Venice does that, and it’s very cool, but it is also has flood issues, concerns about sinking underwater, and was done for geographical and historical reasons starting during the Roman Empire or earlier. Venice was only possible due to its location, a huge shallow lagoon/harbor area where the water naturally provides a good location for that. Trying to artificially get that much water to do that would require huge amounts of effort and resources and disrupt the local water systems. And of course, Venice also has a community with a history of the appropriate skills and cultures that accept the idea. An alternative would be to build new cities where the water patterns are compatible. Though since ocean levels are expected to rise quite a bit due to our idiotic disruption of the climate of our own planet, it might be wiser to wait for coastal cities to start to submerge naturally, rather than building new ones for the current sea levels. Given the future sea rises, it might be a good time for people to start planning for systems to adapt coastal cities to Venice-like operations, though.