You compromise the spacing

High voltage wires are relying on the fact that air isn't that conductive and takes a lot of voltage to breakdown and let a lightning bolt pass

If you get too close to them then instead of the electricity needing to jump through air all the way to the ground it only needs to jump through to air to you then pass through your far more conductive body and make the final hop to ground.

Air will block about 3,000,000 volts per meter of air. We don't run it close to that limit but you might find a 300 kV line with a meter of clearance, but if you get less than 10 centimeters from the wire you're now close enough that it can blow through the air and hop to you just like lightning hopping from the cloud to the ground.

Normally voltages you encounter are low enough that it can't arc more than a millimeter, but transmission lines can get up to hundreds of thousands of volts which is enough for some sizable arcs