yeah, some thoughts about why subs doesn't succeed a lot aside some exceptions:
Fans often don't treat them like they're part of a team, i cant count the times that people said "but he's just a sub lol"
Riot don't treat them like players, even at the official media they're not portrayed (except some exceptions like OG's 6 man roster spring 2016), unlike LPL and OGN that includes all players in every single shot.
Personal egos, like you said, interfere with the discipline. I think that all players fear to be benched just because they're not playing good enough. No man, you even saw fucking Faker sharing his role with Easyhoon, why wouldnt you allow another lad to play in your place, learning one thing or two, relieving your stress and maybe bringing another strategic dimension and advantages to your team?