TANK meta or BOT LANE meta why does it teeter?

I have a legitimate question, been here for 2-3 seasons now and this seems to be what I see(and please no one group try to think I am biased to another XD): Either its TANK meta where moar tanks = wins or BOT LANE (not just adc sometimes) Meta where you pump ALL your effort into roaming(mid) Teleporting(top) or ganking(jung) Bot lane. Why does it teeter so much? I remember the assassin meta but since the philosophy of riot games for assassins is as seen with the Rengar rework: 'If it can one shot you before you get to play the game(countermeasure/reaction) its not letting you have fun' That was broken up faster than either the tank or adc metas ever has been.

I am NOT the best player, but I have always seen bot lane, top lane (tanks specifically) and either jungle or mid assassins as a game of rock paper scissors The best counter for an assassin is an Tank (Rock), The best counter for a Tank is an adc (Paper) and, The best counter for an adc is an assassin (scissors). But a META is like what that rock grOWS TO THE SIZE OF AN ASTEROID AND ALL PAPER JUST BURNS OFF.

<--- meteor covered in paper, result -->

<---- Or an adc Meta is like when that sheet of paper over there.

Is actually compressed (insert metal stronger than titanium) and can cut a PLANET in HALF.

Why is it this way? I know the devs aren't TRYING to make it this way... but why does it teeter back and forth?

P.S. To keep with the analogies Ill add the assassin one... https://cdn.drawception.com/images/panels/2017/5-22/dGg4YS6BBD-10.png (Didn't find any scissors ones!)