What I got from the OP was that he was trying to understand why anyone would consider the 8shots (or 10) to be sub par from a rifle, but just fine in a shotgun.
And while there is no denying that the M1 Garand is a fine battle implement, and a good rifle, there are other rifles today with equal performance down range that have higher magazine capacities.
Several have made the point about the M1 (and SKS) being heavy. And, they are. BUT there is a reason for this. They are designed to fight with. Not just shoot. Not just survive and function after all the bangs and knocks, but to be able to both be useful and functional in hand to hand combat as well.
That's something no one seems to remember, and virtually never takes into account when looking at rifles for personal (civilian) defense. Odds are none of us will ever need a rifle capable of blocking an enemy bayonet thrust and deleivering an effective counterstroke, and then still be capable of fuction and reasonable accuracy.
Modern armies took lessons from WWII, and nations moved away from durable, ruggedly constructed arms in favor of lighter, more easily mass produced weapons. This works for a military, supported by national armories and factories, with replacement systems built in for combat losses.
And those lighter designed and built weapons work just fine for shooting machines, and (legal versions) serve civilian needs as well, as its more than rare for regular citizens to use a rifle in hand to hand combat. Generally, at that close range, we just shoot 'em.
Why do you think all those millions of Krags, Springfields, Mausers, Arisakas, Enfields, etc got sporterized to one degree of another? Not just to make them look better, (although when well done, it was a side effect)but to make them lighter. Deer and varmint hunters weren't going to be fighting with the rifles, all we were going to be doing was shooting them. So we didn't need the extra weight to lug about.