So, are you saying that a 5 carbon ether(which does not contain any O-H bonds) is more soluble in water than a 2 carbon alcohol(which contains an O-H bond)?
Don't care about the Chemisty properties that make it so. For purposes of this discussion, only care about the reality in the physical environment.
MTBE readily and easily contaminates and moves with groundwater. It mixes with water roughly 30 times easier than traditional gasoline components, and does not readily adhere to soil or rock, so it is easily transported long distances. Traditional gasoline components mix much less readily with water, and do adhere to soil and rock, making the ability of the contaminant to travel long distances much less.
Either way, a leaking tank is a leaking tank. The MTBE just made the presence of the leaking tank much more noticeable at a potentially much further distance.