Because it is porous.......or has "pores" in which water can get into...........rather the wood will absorb the water if it is present. Air too.
That's what allows a tree to live........and once it's cut down and becomes "wood" for our use, The same thing still takes place but in a different way.
Water can rot wood and make it unusable....whereas air can dry it out so that it will burn......to heat our home or build a camp fire for example.