The problem with Runes and Masteries
They don't provide outplay potential, just raw stats. Whoever uses those stats better is strong.
Items are slightly different. You can (and should) itemize according to what your opponents are doing. If you're ahead of them, buy more damage, if you're behind, buy more defense. If they have a heavy CC, buy QSS. Still, they would be better if they provided outplay potential instead of just raw stats. Support items are a great example of this. ADC items are an example of the opposite, besides Merc Scim.
If stats are less important, then champions are more easily balanced and the meta can include a whole lot more champions than just the ones who use the stats the most.