Which came first... the chicken or the egg? Why is it a "tank meta" (when it is)... because of tanks>bruisers or because... marksmen too op and we need someone to survive more than 2.5 sec vs them? Depends on which tank meta we refer to. In spring split of this season for instance, it was largely a tank meta not because there weren't viable bruisers or fighters (not all were viable, but some were in good spot), but rather it was because adc's were reigning supreme. Want proof? Just look at TL winning NA LCS or Uzi/RNG winning MSI. Were they that much more dominant than any other team across the board? No. But in an adc meta where it's acceptable/encouraged to camp for bot lane... that is where they look so much more dominant than anyone else. Look at TL this split... not nearly so intimidating. One such reason is that they don't always play to bot side and honestly... doublelift is overrated when he's left 2v2 the whole laning phase. Give him a bit of an early lead though or make him have to 1v2 while his support roams and then you see a better performance from him. But point being... they can't just run a protect the adc comp so easily and successfully, and it shows.
Fighters and bruisers should win lane. Less so fighters who hyperscale such as jax or fiora who should honestly require about 1.5+ items before they start being able to win trades vs tanks. If tanks play safe, don't shove the enemy tower, and just farm up, they should be behind in cs but not much behind in xp. Due to cheaper items for them, the tanks should be on about equal power in early/mid game teamfights.
Mid game is when fighters should really start ramping up their ability to splitpush... with bruisers CONTINUING their splitpush advantage vs tanks. Fighters will still be too fragile to teamfight yet unless they built their defensive item(s) first in which case their splitpush advantage will be minimal or none. As some of the best have described bruisers vs tanks... one is tanky and uses cc to peel enemies or lock them down with an engage and hopes to disrupt the enemy's damage while the other dives in to try to delete key targets. Both should be about equally powerful in teamfights if executed properly. A tank should have some meaningful burst damage... not enough to nuke 1/3 of someone's health bar, but more than a janna's auto attack worth of damage. They are slightly tankier than bruisers... not much, but some. The key difference between them and bruisers at this point is their cc abilities which can enable them to peel or lock down key enemies for their team while avoiding being bursted themselves.
Late game is when fighters are unrivaled at splitting, dueling, and in small skirmishes. They are also tanky enough to get involved in a teamfight and do some serious damage if they are not focused down by enemy cc and burst. They are still more fragile than bruisers though... leaving them vulnerable if they are the primary focus/engage of a teamfight. Bruisers split the difference between fighters and tanks... some being tankier with less damage and others being more damaging but less tanky. They still win a splitpush vs a tank, but not quickly enough vs most tanks that they can exploit it easily. If they itemize purely for splitting, they can probably be as effective as a fighter splitting vs a tank; however, they sacrifice their teamfight durability to do so. Likewise, they will lose their ability to nuke out enemy carries in one combo or so at this point of the game depending on itemization. Tanks are extremely durable at this point and no class can burst them out by themselves. They have durability to hold up for a good while vs enemy bruisers and fighters as well as in teamfights. Their durability is notably stronger than bruisers at this phase of the game due not only to itemization, but also to their innate stats/abilities. Tanks' damage at this point in the game should be more negligible in most cases.
