Meddler has already explained why Sej is so OP.

He admitted they won't need Graves or Lee Sin. Their solution was to buff other champions to raise them to that level so don't expect a Sej nerf soon. Her numbers are OP on purpose.

Isn't it obvious? RIOT like the Jungler meta. They want you to win or lose based on the most snowbally jungler. This isn't going to change.