Balsa actually has a very good strength to weight ratio. So does aluminum. The difference is that in order to take advantage of aluminum's good stength to weight ratio, you need to get it into the right shape, which is harder for us to do with our tools. This is why you see aluminum tubing and C-channels used, because they have already been shaped, using methods we don't have available, into useful forms. The other problem with aluminum is that getting really good joints is harder using the methods we have readily available. Balsa, on the other hand, delivers its favorable properties in 'bulk' form, so that it only needs to be cut to shape. Since it is so light, thick pieces of it can be used, so that good glue joints are fairly easy to attain. It is unlikely that most of us could take advantage of the inherent strength to weight advantages of aluminum, because we can't get it in the right shapes.

Modern aircraft that are built in garages are indeed built largely out of wood. Modern aircraft that are built in modern factories are built largely out of aluminum, or other, more exotic, metals.

Plastic, specifically coroplast, is somewhere in between. It has really good properties in terms of strength, weight, and joining methods, but it does not allow us to get exactly the shape we want, as we can with balsa. If you are OK with the shape you get when you bend coroplast around a curve, which is fine for most purposes, then it is a great choice. You probably won't see sailplanes that depend on a really accurate airfoil using coroplast. They will be built using materials and methods that allow much more precision.

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