Too much armour pen/reduction

Last few games I've had where we've had 2-3 tanks vs full ad teams us tanks with 300-400 armour and 3-5k health still seem to just get melted, what's up with that, it never used to be like this? The ad teams don't even need to flank and dive the backline they can just go head on. Any tips ? It seems like there are too many armour piercing items that couple with plenty of champion's ability to do the same.
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