For it to absorb moisture from the atmosphere it has to be exposed. In a sealed container or a petrol tank the volume above the liquid is fuel vapour, there is effectively no "air" in a fuel tank, and thus no atmospheric moisture. Modern fuel tanks in cars are not "open" to atmosphere, they will pretty much always have a slight positive pressure of vapour.
Degradation by other means is a different issue, and stabilisers can help with this. The best solution is to either empty it or fill it completely, bulk fuel degradation is significantly reduced. Carbs tend to be the biggest problem, small jets and open to atmosphere. I usually drain the carbs and fill the tank on bikes for winter.
Mowers are easier, just drain everything, usually not difficult. If I am called to a mower which won't start in spring, I usually just try some brake cleaner down the intake first, the issue is often simply lack of volatility to get it going. Once fired it will often continue on the old fuel. Then just add as much fresh fuel as you can and it's usually fine. They usually only form the dreaded jelly if left for years rather than months.
