The resonances of a single narrow tube are simple and well separated. The frequencies are all multiples of the fundamental frequency. Joining several tubes end-to-end does not introduce more resonances, compared with separate tubes, though it does change the resonant frequencies.
For most instruments with soundboards, the soundboard itself is two-dimensional, often has a complex shape, and often has ribs or other features to strengthen it. Even for a simple rectangular plate, the resonant frequencies are proportional to (m^2/a^2 + n^2/b^2)^0.5 where a and b are the lengths of the sides of the plate, and m and n are integers 1,2,3,...
For a square plate, this gives frequencies proportional to
1.00, 1.58, 2.00, 2.23, 2.54, 2.91, 3.00, 3.16, 3.53, 3.60, 3.80, 4.00, 4.12, etc.
and the higher frequencies are even more closely spaced.
These resonances are much more complicated than a pipe. In fact soundboards are usually intended not to have distinct resonances, but to resonate uniformly at any frequency.
This shows the measured response curve of a piano soundboard, over the frequency range 0 to 5 KHz - taken from https://www.isma-isaac.be/past/conf/isma2010/proceedings/papers/isma2010_0202.pdf (which has more experimental data).