Poppy doesn't always win her games, but she kind of always wins in her own way. How many times during World's did we see teams pick Poppy into losing match-ups, end up down 40+ cs, and yet as soon as she builds Sunfire she becomes impossible to deal with regardless of how rough her laning phase was.
You can't gank her. She can knock the entire team away, charge off, speed herself up, and slow you on top of being generally difficult to kill in the first place. You can't starve her because all she needs is one item and one good W, E, or R to shift an entire fight. Even the go-to anti "wall of stats" champion Trundle can't do anything to Poppy. She stuns him against his own pillar and runs away while having more presence in 5v5. He can't even touch her 1v1 with Subjugate up.
She's incredibly tanky and brings lots of crowd control along with respectable damage. She's really good at disrupting, really good at making picks, really good at disengaging, and really good at teamfighting. So how exactly is Poppy meant to be kept down? What other tanks can compete with her? The only one I can think of that comes close to bringing her level of disruption and mobility is tank Ekko.