'Zero gap' Mandibular Advancement Devices (MADs)??

Does anyone else find it strange that pretty much all MADs are built such that there is a big gap between the top and bottom teeth? (I'm talking about the vertical gap ie the difference between having your mouth closed vs open.)

When you sleep normally, your mouth is closed and teeth close to touching. I find it really uncomfortable to kind of have to have my mouth 1cm open to use the MAD.

I don't see anyone else ever talk about this though?
And has anyone found close to a 'zero gap' MAD available in the UK?

I get that they build this gap into some, as some people mouth breathe - but we don't all mouth breathe!!