Petrol, is a volatile liquid, which at normal temperatures, is just on the point of evaporation - which basically means it can be either a liquid, or a gas. It doesn't require much pressure, or much concentration above it, in a gaseous state, for it to be maintained as a liquid. In other words, it doesn't need much, to keep it in the liquid state.
Which explains why, despite tanks having a breather hole, in the past, the petrol didn't instantly evaporate. Anymore than you leak at the filler pipe, caused your entire tank of fuel to evaporate.
My tractor had a full tank of fuel, when last used September/October. I pulled it out just yesterday, to inflate the tyres, and check it over, ready to cut the grass, for the first time later this week. The tank has an open breather system. The fuel tank was, as expected, still full yesterday, no measurable evaporation I could detect.