Because a plenum that is too large can hurt turbo spool as well as hurt off boost performance. Short runners are desirable for high rpm power, for example the JMF length runners would be useful for an engine revving to 10k and up (which applies to no one) so it's not really optimized for our engine.
The absolute BIGGEST mistake that people make when it comes to intake system modifications and their flow is that they have the mentality of "if some is good, more is better, and too much is just enough". An intake manifold's flow should be matched well with the cylinder head, the reason for this is because usually if something flows more, it just means that it's larger. Bigger isn't better because larger ports have lower air charge velocity, lower velocity means less cylinder filling and less cylinder filling means less power under the curve.
Shorter runners are good for high rpm power because it makes good use of intake valve pulse tuning and longer runners are good for lower rpm for the same reason. Plenum size isn't nearly as important as runner length or diameter but still plays a minor role and that role is magnified when applied to a boosted engine. Lastly, plenum size *usually* does not affect cylinder flow balance so for them to alter it wouldn't yield much results.