Tank Meta - WHY?

So maybe it's because I'm not big on top lane and I'm pretty mediocre in the jungle, but as a mid lane main I find this current tank meta extremely problematic and frustrating. I'm not sure why it's totally acceptable for a champion in this game to build tank items and still do crap tons of damage and CC, when usually in video games it works the exact opposite. Now, I know Riot wants to make playing tankier champions less one-dimensional - that's great - but why can champion after champion literally just run into me with zero skill, hard cc me for a year, and then 3-shot me with one item built that doesn't even do damage? How is it healthy for the game to have a sizable group of champs that no one can kill, no one can get away from, and no one enjoys fighting against? And as far as building against it as a mage? Good luck. Liandry's is a joke. The only "anti-tank" item particularly available to us and it does jack crap, even after the little buff it received a few months back. (In a similar vein, why is Liandry's 3000 gold and Void Staff only 2650 when all you're getting differently is HP and a crappy passive that doesn't accomplish what it's actually supposed to accomplish?)

Just wondering if someone can explain why this is the only game I've ever played to operate like this. I've been playing video games and RPGs since the Clinton administration and I'll be damned if I've EVER seen a character be allowed to build really tanky AND have a lot of utility, mobility, and damage. This season has me hard-tilted like nothing else. I honestly have no idea what Riot's balance team is smoking every day, but I'd love to be in on it.