Why are the easier champs to play the strongest?

You are in Silver 5. That is not an MMR in which skilled players congregate.

Champions who are simpler and more straight-forward are more difficult for unskilled players to screw up, which would lead to them performing better than their more difficult counterparts.

It doesn't matter how powerful a champion is if the player isn't skilled enough to actually use them competently.

In addition, simpler champions are more reliable while more complex champions are more flexible in most situations.

Also, I'm surprised you are complaining about simple champions being strong when Nasus seems to be your most played champion. While I love Nasus and have him as my most played champion in S5 and S6, he isn't really that complex.

imo sure they have a lower skill cap than other champions, but a player with great knowledge of a champion will be able to outplay and succeed with that champion no matter the skill cap.

Because since they're so strong, they can't really get any better since they don't have a lot of potential or room to grow.

Now when you look at they are way more difficult but will end up being stronger in the long run because they can do so much more than just spamming Q W E and R. It's really easy to see through the easier champs techniques because well, they can really only do one thing. People think fizz, which is one that you listed, is so broken and overpowered, but really he only has one technique and he's easy to beat once you know it. Same thing with Lux, Veigar, and Annie...and blitz is useless if you're good enough at dodging skillshots and knowing to stand behind minions/know when he's looking for an opening.

Lets see you land every single pull and disengage at the right time to save your adc's ass.

"I really hope that my enemy doesn't know what the extra bar under my mana means.

"Why don't you just cry about it?"

Bruh you don't even know how much skill it takes to get all that passive AP and press r in a team fight. "who cares if i have 6 rings if i have 800 stacks"

Each champion has their own skill cap. So each champion can be played at a certain level of mastery. Some people like kits of champs that are not complex. In cases like with overloaded kits, people dont get them or just cant play them. The simple kit champions are not OP. They just play a champion they know well that they can carry with. Everyone has their mains. So every champion is OP in the right hands.