High-voltage DC is actually more efficient for long-distance transport, and makes it easier to interconnect grids. Efficiency is not why we use AC. Installation

High-voltage DC is actually more efficient for long-distance transport, and makes it easier to interconnect grids. Efficiency is not why we use AC. Installation cost and history is, because you need converters everywhere you connect it to an AC grid. Up until recently a lot of standard pieces of equipment weren't practical for HVDC lines. Electrical breakers, for instance: They existed, but tended to have short lifespans. That's because DC lines never go to zero power, whereas AC lines do so a hundred times per second. Modern technology has changed that. The equipment is still more expensive, but that would be made up for by the reduced transmission losses... if we were starting from scratch. We're not.