99% of players will ignore you because they do not know mechanics or want to know them.
...Alternatively, it could be that condescending "I know 10x better than you, listen to me" attitude. For real, trying to claim that the reason people don't listen to build suggestions because "they don't know mechanics and don't want to know" is asinine, and, ironically, exactly the kind of condescending nonsense the OP is pointing out.
Besides, there are multiple potential reasons for people to not listen to the suggestions of others. Maybe they just think the person is full of it, maybe they just want to try their own thing, or maybe they're confident that what they're doing works.
Trying to act like they're just ignorant dumbasses who don't understand or want to understand anything is just plain dumb.
I understand the urge to be polite..... but I also think politeness needs earned.
And nobody would be inclined to try to earn the politeness of some random person they'll likely never meet again. No disrespect, but you can keep your politeness, as what matters more when offering advice or criticism is respect and tact.
As an example, if the enemy team has a lot of magic damage (Sona Support, Xerath Midlane, Diana Jungle) - it may be easy enough to advise "ADC get Maw" etc. - but, it could be much more effective to say "we might want to start stacking magic-resist."
Why would that be more effective? Because each player would get the suggestion (buying magic resist), have an incentive to do so (not getting nuked by Sona, Xerath, Diana), and each of them would be able to approach it in their own way.
The ADC could browse Attack Damage items that have Magic Resistance (Mercurial Scimitar, Maw of Malmortius) and plan their picks accordingly - are they dealing with burst damage? Maw. Are they dealing with CC? Scimitar.
The same rough self-imposed search could work for any role. Tank? Abyssal Mask. Gargoyle Stoneplate. Spirit Visage. Fighter? Maw of Malmortius. Mage? Banshee's Veil. Assassin? Maw. Support? Locket of the Iron Solari.
And, ultimately, if someone doesn't buy something you'd want them to buy - understand that there's likely a reason for it. Maybe they wanted to commit to a certain build path, maybe they figured everyone else had enough to compensate, etc.
Just don't be that asshole who starts ripping into someone for not buying the items you want them to buy.