Unfortunately there’s a lot of Mis-information floating around out there. Polyurethane does not seal out all moisture transfer, nor does any other commercial wood finish. All finishes do is slow moisture transfer. Also as mentioned wood does not “breath”, nor does it need to exchange moisture necessarily. In fact the ideal state for a guitar or a piece of furniture would be in a climate that never changes, meaning the wood neither acquires or releases moisture. As most of us don’t live in such environments our wooden objects do in fact gain and lose moisture with environmental changes.
Now as far as using narrow boards to glue up table tops, the two main reasons for doing this are for economical and/or lack of understanding. A 4” wide board does not move less in proportion to its size than an 8”. In other words if you take an 8” board and cut it into 2 - 4” boards, then glue them back together the same way it’s going to move just as much as before it was cut. So manufacturers “cheat” by flipping the boards. This minimizes the affect of cupping by alternating it up/down, up/down. I’ve spoken with many guys over the years who utilize this method without understanding it.
As for finishing the bottoms of tabletops I’ve seen it done both ways. My opinion is that for the most part it’s done for financial reasons, cheaper not to finish the bottom. I’ve done a fair number of solid wood counters over the years and always finish the bottoms. Never had one split, but as a woodworker I probably pay much more attention to the quality and condition of the individual boards I use than most bigger manufacturers.
Guitars seem to attract a lot of interesting mythology. As a woodworker I think many commonly accepted ideas are more salesmanship than anything else, from tonewoods to finishes. In regards to the original question I don’t have an answer, but my guess would be a combination of tradition and cost. The finish on a guitar is really about protecting the wood, (which is extremely soft in many cases), from everyday dirt and scratches. Anyone who has handled a raw cedar or spruce top understands so much as looking at it funny will scratch it, so the finish protects it. Whereas the interior does not need protection. No need so no reason for additional cost😉
Good luck,
Jeff
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