The body adapts well to all kinds of physical stresses, but each kind of stress induces a slightly different set of adaptations and not all adaptations actually lead to a high level of muscle mass - high volume physical workload increases mostly the endurance capabilities of the muscles used, and to a minimal degree strength and muscle size to match the work done, but it can only do this to a point.
If you try doing both together (high volume high frequency and high intensity), you will develop only to a certain point in all things before the overall stimulus and resulting stresses become too big for the body to further adapt and you either stagnate or hit overtraining syndrome (and not just stagnation but regression).
Workers like labourers develop a bit of muscle and some good endurance capacity to a point, but adaptation always stops at a certain level (much below the level you'd halt on if just trainig for one kind of adaptation), and this level dictates the maximum effort that person can then work at. Since no labourer either develops the endurance of an average marathon runner, physique of an average bb'er, or the strength of an average strongman, it shows that this kind of activity doesn't lead to optimal adaptions in any one particular area, it simply leads to adaptations to do that particualr job 'a bit better' than before.
Best way to train for size is bodybuilding style training, best way to develop strength is strongman or PL style training, best way to develop an all round functional physique is to use a mix of training techniques... but with the thought always in mind that you can never look like a bodybuilder, be as strong as a powerlifter and have as much endurance as a marathon runner by training this way - you will develop an excellent and above average range of physical capabilities, but won't achieve close to your genetic potential in any one particular aspect.
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