Since you mentioned it, I would be perfectly ok having an alcoholic beverage while playing normals, while I have a strict no alcohol policy when I'm diving into ranked. Normals are a big mishmash of skill levels in the same game, so a slightly intoxicated me (who is high plat) is probably still going to win against most players (since most players are gold or below) even if they were trying their best, and so theres no harm in it. In ranked on the other hand, everyone is theoretically around my skill level, so even a slight impairment is going to make it much more likely that I'll get outplayed.
Whether you think people 'should' or not, the reality is people aren't playing their best roles and best champs in normals (and where would you suggest people learn new champs/roles if not normals?), so people don't need to play at their best to keep up. There's also no limitations on premades, and ones ranked mmr has almost no correlation with their normals mmr, which can result in matches that would be extremely lopsided if everyone played at their best as it might be a team of silvers versus a team of diamonds.
In ranked, people wants to win as quickly and as effectively as possible. In normals, people don't necessarily want to win as quick. They generally want to win, but once they establish a lead, many people are more inclined to keep farming kills and trying to achieve their full build unstopppable fantasies, rather than pushing down the nexus.
It's also extremely fun to make things more interesting through 'handicaps'. If I find myself in a premade with a bronze player for some reason, I will pretty much never play janna support, since it's not fun for me since I'll completely run circles over everyone as someone who mains janna and probably plays her at a level beyond my rank with over 1 mil mastery points. I will however play janna adc or mid or similar, and I'll probably still do ok. It's fun for me because it's rediculous, it's probably funny to my teammates once they see I'm doing fine (since I main the champ and can pull it off), and it's probably more fun for my opponents who may have a chance compared to if I was playing a meta champ. Everyone is having more fun, so it seems strictly better than alternative approaches to normals.