No drain in gas tanks?

Well, some vehicles do, but it adds complexity, weight and cost.

As for why they are not fitted as standard, then that becomes more involved. You say that one humid day and one night was sufficient to cause enough water - I would suggest that the water had been collecting over a period of time and when you parked on the slope was sufficient to then be drawn into the pump etc.

One way of avoiding this is to keep the tank between half and full reducing the volume of atmosphere in the tank over time.

As for the comparison with airplanes - a car has a fuel tank of about 70 to 120 litres, the A380 has a full fuel load of 320000 litres which if you start to think about a 0.01% water contamination is something to be considered, especially at 32000 feet... Fuel contamination in a car on the side of the road is not the same thing...

Some cars have a fuel filter combined with a sediment / water trap which can easily be drained. This can be an easy retro-fit if one is having water contamination problems - and these can be due to some filling stations of dubious quality...

I have never seen a drain in an auto tank. When I had a problem , I pulled out the tank and cleaned it . It was not difficult, an older, full size Olds. I also drilled a hole , drained a tank and put in a self tapping screw but this is not a long term condition.

My 1971 Datsun 240Z definitely has one. I think all the Z's from 1970 - 1978 had drains. Can't confirm with absolute certainty. So, some older cars have drains.

If you can drop the tank without removing it completely you can usually get access to the inside of the tank through the fuel level sender opening.

Probably the same reason so many cars have a hose-block in the fill tube; To make it harder for people to steal gas.