Can someone explain Fighters and Slayers to me in toplane

Fighters - Juggernauts/Divers Slayers - Assassins/Skirmishers

Why does get to be as tanky as a juggernaut but dish out much more damage? I really don't understand. Shouldn't they be squishier in comparison. I feel Fighters should be able to abuse slayers in lane and bully them while outside of the lane these champions really show their strengths with some exceptions and outscale.

For example, all three of the above champions outscale a Fighter with one item.

When you think about it, shouldn't the class roles be like this:

• Fighters/Tanks counter Slayers
• Slayers/Fighters counter Marksmen
• Marksmen counter Fighters/Tanks
• Mages dish out AoE damage
• Controllers support the team

Then you have very few niche champions that can shine in other areas but not as well as champions designated to that class. However, it's like Conqueror threw this whole system out of loop. Skirmishers are supposed to be heavily item-reliant like ADCs. To be honest, I think Yasuo is the best definition of a melee skirmisher because he's very item reliant compared to the rest. With their mobility, they should viable as ADCs as well.

How come we aren't going with this paradigm?

P.S. This is to incite discussion