Sivir is currently in better standing than Vayne but only has a slightly lower pickrate.
It's the same thing with other champions. If it were Ashe, there'd be an excuse how good/healthy she is and it's just the meta making her like this. But many remember when Ashe was OP how agonizing it was to be slowed+CC chained eternally while being out of range to counteract her.
Every. Champion. Is. Unhealthy. When. They. Are. Overtuned.
Sivir is busted and has been strong before the patch. She's just been sleeper OP like Vayne before Runes Reforged.
Her W buff and the high mana regen she gets from her E coupled with the ability immunity, her strong scaling, teamfight potential, and lane poke makes her significantly strong and hard to deal with.
She may have a lack of escape, but her R is pretty low on CD with item build paths+runes and her E is effective. Sivir simply needs to be with someone to pop off and she will always be with another player as her kit excels really well with 2 or more allies.