Why doesn't the system memorise summoners/runes/masteries for each role seperately?

Different game modes already keep track of the last set up used in that game mode. However, with dynamic queue, we can extend it even further to each role.

It's very annoying if you forgot to switch to smite as jungler, heal as adc or exhaust/ignite as support. In blind pick, this is not a big deal. You'll underperform, but you can still have a good game even if it made you lose. However, in ranked, you want to perform optimally and it's really frustrating if you notice in your loading screen that your summoners are incorrect because you got your secondary role last game. I haven't experienced this in ranked myself, but I've heard about others who have.

It may not be a huge deal, but it are the small things like this that separate the good games from the great games.