Furthermore, it is mentioned in some sources that Storm Bolters, though having a high rate of fire, aren't fired in long bursts because they lack the ammo for it. Both in the clip and on the Terminator wielding it. Surely they can operate buttons though, because I can operate buttons on pouches with winter mittens on and those are more cumbersome than Power Armor gloves.
It's not about why they couldn't have it all with them, rather than the sheer insanity of modelling the damned pouches on every single Marine and thus breaking the pristine surfaces that are reserved for chapter iconography and livery. The only SM models that really have a reasonable amount of equipment on them are Scouts:
http://i43.photobucket.com/albums/e3...Gallery441.jpg
And even they pack light. Despite their worse skills in combat, I doubt it would be such a radical difference that it would make it logical to have one Marine with one magazine be equal to one Scout with half a dozen spares.
And for quadruple stacking magazines? Those mags would be mechanically extremely unreliable and also very VERY wide. In the BL books, Marines are reloading their weapons from time to time, so obviously they do so in "real combat". Just accept it as a modelling defficiency, because that is all that it is.
For fluff reasons, they would carry at least a half a dozen spare mags for something around 100 bolter rounds per marine. Then the Rhino has some more. Heck, even SM missile launchers only came with 2 Frag and 1 Krak missile back in the day. Why could they fire 6 turns when the maximum ammo was 4?