A reason? Well, Sett is easy to figure out after a few games and people haven't learned how to lane against him. He's not really that bad to play against if I short trade or dodge his W. Sett can be a bit predictable, yes? I've won majority of the Sett lanes I've faced but I've also won lane as Sett majority of the time. Huh, funny how that works.
But if you want a better reason, its because his W has true damage and a decent shield and the damage area is a little bit big. It's not that hard to dodge in lane because it takes a bit to go off so I can react to it even with Hawaii ping, but its very spammable for lane (in teamfights, the cd is long enough so he might barely get two off if he's in the middle of everything). If you challenge Sett, his W punishes more and his passive is just better than Garen's I guess so its harder to kill him through short trades alone. Very sad that Garen is being left behind, he's weak right now (even in low ranks) and Sett just steals his passive and made it better.
So yeah, tweak Sett's W, he'll be fine. The rest of his kit feels a bit reasonable, just as I said before, his W might be too op. Don't let Sett get his W on you and punish him off cooldown. His W has high cd at first, deny him cs early and everything. In teamfights, just like facing an AP Malphite, don't group up so closely when you see Sett with his ult up. A lot of champions in this game can be annoying and op in a way, but you learn how to face them, like Teemo's annoying damage or Fiora and the constant wall hugging.
https://na.op.gg/champion/sett/statistics/top ....Why is Sett facing Soraka in top for 80 games? Huh, weird. Anyways, it seems he wins in majority of the matchup stats. Kled, who I assume would do well against Sett, just barely scratches through in lane kill rate. Sett beating Darius is no surprise to me (even if I won against Sett as Darius in every match I did, I still think Sett should be favored). Sett's W currently makes Sett too forgiving to play in lane along with his buffed Garen passive.