Ya, I'd like to read a few possible answers. And you're confusing me not sugarcoating things with me throwing out insults.
So to answer question #2, some mystery that alludes you:
"And also, why do they often claim that their own magical beliefs are more sophisticated than the magical beliefs of others?"
First and most importantly, is basic human psychology and sociology. The same thing that drives you to wear clothes to work or to speak in cogent sentences instead of using random symbols that only have meaning to you: There is an inherent trust in things that are
your own, whether they're your own style of clothes, your own use of language, or your own belief (or non belief) in religion. In other words, regardless of what your belief is (even if it is non belief), you presume that you
are correct. This could also be the foundation of law, civil society, and so on, a general assumption of those participating in being correct, versus say, chaotic madness, and even then, if you prescribe to madness, then you probably also believe that it's correct.
But, beyond psychology and sociology, which I think play a far stronger role, is internal consistency. This is something that is sometimes lost on the insult atheist, but most mainstream religions in the world have fairly strong internal consistency, that is, that ideas, philosophies, behaviors, theologies, and practices are
fairly consistent with one another internally. Now, I am not particularly religious, but I've got a fairly strong theological understanding of Catholicism, Judaism, and Islam; I say Catholicism more so than Christianity because I simply haven't studied most Protestant Christianity beyond the basic history and basic beliefs, and even then, my experience with most denominations of protestantism are in how they are different from Catholicism or Orthodoxy, not so much in their internal philosophical machinations. Generally, Catholicism, Judaism, and Islam have rich historical traditions and deep histories of philosophy tied into them, all three grew during profound periods of philosophy in their respective cultures, and they tied in various elements of secular philosophy along the way, strengthening them. This isn't necessarily a detriment to newer faiths (even newer protestant faiths, many of which are still hundreds and hundreds of years old; and to some its even an asset), but through through thousands of years of development, those three religions are able to generally 'boot out' heresies that would make them internally inconsistent. So, while, your sister's care bear religion may look very convincing to both you and your sister, it probably lacks the rich historical tradition that develops it into an internally consistent world view. For instance, if your sister is the apex of the Care Bear Religion, the head of the church, you might be able to trip her up on a consistent care bear philosophy rather quickly (within one or two questions), but you wouldn't be able to do so with, say, Cardinal Ratzinger.
So, to answer question 2, I'd say it's both human psychology and internal consistency.
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