That's an awful lot of assumptions.
I don't offer advice very often because people don't want it, especially in ARAM, which is intended to be a casual mode.
I also find that the vast majority of "constructive criticism" is anything but. Like you, I would be grateful as hell if someone said, "Hey Taric, I think one of us needs to get a Frozen Heart. Are you going to pick it up or shall I?" What you see, however, is a lot of bullying and harassment over item choices. I don't generally see much toxicity in my games, even as a support main, but having people try to micromanage my build is often a thing.
In addition, telling people how to play - even gently - often comes across as arrogant and condescending. People who are not me often have different perspectives and less knowledge of my champion. Let's say I'm rushing CDR cap to supercharge my Ardent Censer for objective control on Sona, which is shockingly powerful and difficult to counter. When someone comes along and complains about my build, it's not easy to have much respect for them. This is a considered choice I'm making and I frankly don't have the time, mental bandwidth or the inclination to have the conversation in the middle of a match.
I agree with you that it should be much more commonplace for everyone to be decent with each other, but I don't think expecting people to routinely accept criticism in the middle of a match is a necessary part of that.