"So what's the theory as to why it's out in the desert?"
I don't think Noah built the first boat ever. I think there were boats on bodies of water, whether ocean, lake or pond.
The fragment found in the Kuwaiti desert means the same thing as oceanic fossils showing up on mile-high mountains ... a universal flood.
As for fossils on mountains, they are there because of geologic uplift, as can be seen by the sharp angle of the bands of fossilized sea bed. Clearly, when the bands were made, the area was flat seafloor, not mountainous, and the layers were made over a very long time period before the uplift began. The fossil sea critters now seen in mountain rock are of course far older than the boat in any case. Wrong time period to be concurrent with the boat, unless you're thinking multiple floods.
The fossils, maybe. A boat could have ridden in on a tidal wave.