Birds and utility workers are safe if they only touch one wire and nothing else. It's not voltage, but voltage difference that's dangerous, just as it's not more risky to step off a curb in Denver despite the mile-high elevation. Large birds like eagles are at risk of spanning two wires, and many die needlessly. It's more efficient to transfer electricity at high voltage and reduce the level before it enters our homes. Different designs for power lines reflect the different loads they carry.
Power is expensive, so it's important to minimize loss when moving it around.
You can put your thumb over the end of a hose to make a jet, but it will take longer to fill a bucket because there's less water at greater pressure. So too, electric power depends on both voltage and current. I 2 R Power loss depends on voltage and resistance and the square of current, so it's more efficient to transmit power over long distances at high voltage and low current. High voltage is dangerous because it can jump between conductors, so high voltage power lines must be widely separated. That's effecient, but expensive.