Why are Juggernauts' weaknesses subverted by their core items?

Juggernauts' weaknesses of having to get in melee range and being slow are completely subverted by their core items. Black Cleaver and Deadman's Plate completely alleviate the Juggernauts' slow hulking nature and give them almost constant uptime on being quite quick. While items like Sterak's Gage and Titanic Hydra are problems for being hyper efficient tank/damage items; the mobility granted by Black Cleaver and Deadman's Plate subvert the weaknesses of Juggernauts.

Most other classes in the game have their weaknesses partially alleviated with a high downtime, such as Zhonya's Hourglass for mages; where the active keeps them safe from burst and allows them to wait for their cooldowns to be up. Zhonya's active has a 90second cooldown though, meaning using the active is impactful but it has considerable downtime, therefore using it is a big decision and mages and assassins are weak while Zhonya's is on cooldown. Juggernauts don't suffer from this long downtime on their weaknesses being alleviated though. Juggernauts are almost always sped up by either Black Cleaver or Deadman's Plate, meaning if they're able to get ahead and get their core items, they're very hard to deal with.

Juggernauts being high damage tanks who run at you isn't necessarily a problem. Shyvanna and Mundo can run amok in team but are managable. Having very high uptime on being sped up is a huge issue though, because there's no window to capitalize on the Juggernauts being immobile tanks. They just sprint at you and deal incredible damage while being incredibly tanky.