I've read -- though I don't know how much truth there is to it -- that the pressures from the strings and bridge on a flat-top guitar can cause a plywood top to delaminate. Grains running different directions, stresses running various directions, and the two not interacting well.

It is notable though that some quite pricey archtops have plywood tops.

Plywood back and sides...if you're holding that guitar against your body while you play, it's not going to resonate anyway, is it? Not much point in going solid wood there, maybe.