Because the pods are all the stuff inside the armor shell with one surface that is variable b/c it is what seals up the opening for firing.
This part doesn't make much sense to me, as there are weapons that obviously cannot fit certain places and locations in some 'Mechs, and there are weapons like Arrow IV that most likely need to be split across two locations, a Vulture V cannot cram an AIV in its right torso bay alone, not stats-wise, probably not physically.
And fiction/fluff actually describes OmniMechs looking radically different depending on the configuration. The Vulture C was noted as being thought a different OmniMech at first by Spheroids on the account of its torso bays being very different from Vulture Prime, per description of TRO3050 (original) (though that one implies the Gauss rifles are in torso and ammo in arms, while the record sheet has always placed the guns in arms). Incidentally TRO3050U describes the Vulture's torso bays "being practically modules themselves", implying some configurations omit or alter them radically.
Were pods purely internal, these descriptions makes no sense whatsoever.
I can grant that miniatures are effectively artistic license... but even official art does depict things different. The WoB Celestials have three pieces of art each, one for Invictus, Dominus and Infernus configurations. These all have differences that alter the outline of the 'Mech rather than fitting everything within Prime/Invictus-shape.
Then there are melee weapons, that are magically protected by armor but surely aren't fit within existing housings.
