We do because we can

The world’s thinnest mechanical wristwatch may have only 167 parts, compared to the 269-part tourbillons and 407-part minute repeaters his colleagues contend with, but the fact of the matter is that ultra-thin watches featuring nothing more than the hours and minutes qualify as extreme complications in their own right... More here.

It is hard to believe that a 2mm thin mechanical watch is possible, but it is. Still makes little sense to me though.