This game honestly feels like, which ever team has more tanks wins.

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How a champion builds isn't what determines whether or not they are a tank.

As an example, I could build Vayne with 5 defensive items and she would still not be considered a tank. At most, she would be a durable and low-damage marksmen (bordering on ranged fighter).

The simplest way explanation would be the difference in how the two roles threaten the enemy team:

• Fighters are threats primarily due to their damage.
• Tank are threats primarily due to their utility/CC.

Volibear generates threat primarily due to his damage; if you let him get within melee-range of a squishy champion, he is going to kill them horribly. Outside of his damage, he offers little utility for his team (those being a short-range AoE slow and a melee-range fling.

Shyvana has minimal utility (limited to her knock-back on the ultimate), but she will beat your face in if she can stay on top of you. She is a fighter whose kit (similar to Dr. Mundo) provides a large amount of innate damage and little team utility.

Sejuani is a tank whose primary threat is her CC. While she does a decent amount of damage, her kit provides a large amount of disruption, initiation and lockdown. While she may not necessarily killing you single-handedly, she will keep you locked-down long enough for her team to kill you.

How a champion build does kinda determine whether or not they are a tank. If a champ is stacking full on health/armor/MR then they are considered a tank. Doing that on vayne makes her a tank whether you think it or not. That's why we have thing like tank ekko even though he is meant to be a mid lane mage.

Tanks are threat not from their utility/cc alone. Tank are threats because they are hard to kill and will do more damage if they aren't killed fast enough. If what you're saying is true then Mundo is not a tank because he only has 1 CC and no utility and the Juggernaut class is not a thing and champ in that class should be classified as fighter instead.