Why is tobacco so moist when you buy it? Is it to allow the flavours to marry?
Partly it's to promote marrying of the flavors of the component tobaccos. Partly it's to help against the tobacco drying out in the tin before the tin is opened. Keep in mind that few tins are actually sealed 100%. They're mostly leaking very slowly from the moment they're sealed. Cutter tops were great for long term storage, seven years and beyond. Square and rectangular tins are not.
Depending on where you are and the season, indoor relative humidity is around 40-50% so tobacco left to air dry will not get any dryer.
That can be an issue in many locations. If you have a desk lamp with a flexible neck you can place a dish with tobacco near the light source to gently evaporate the moisture. Years ago there were tobacco warming plates one could buy for just such a purpose. And you can zap it in your microwave, though I personally don't like the effect superheating the tobacco has on it's flavor.
I should be able to just leave the tin open? What am i missing?
I don't see that you're missing anything, but leaving a tin open in a highly humid area isn't going to help it dry. I live in a dry climate, so opening a tin and letting it dry works quite well for me as long as I remember to stir the tobacco in the tin every 30 to 60 minutes or so to redistribute the drier leaf with the moister. You can also leave the top slightly ajar, which will slow the drying process, but help mitigate against the top level of tobacco drying out too much before the rest of the tin dries down.
In any case, it all depends on what you want your smoking experience to be. Indoor smokers have an easier time of it because they're sitting in a cloud of their smoke that they can smell, so the level of technique doesn't have to be all that high. Outdoor only smokers need to have a higher level of skill if they're going to taste what the blend has to offer. That's when moisture levels become more critical.