One Piece Episode 480

[Splash!]

They don't interfere because the pacing has been dragged out alot compared to the Manga, where these events flowed much much faster than they have here. Additionally, everything that is happening in the war isn't always being show on scene. Like for example, what ever happened to Crocodile and Don Flamingo's fight? Obviously they did fight each other, but they simply chose to not show it. Same can be said about alot of stuff potentially going on. You never know who/what could have been holding up the Admirals, or if they would have even had a chance to interfere.

This is exactly it.

The anime has dragged out events in this war to make it seem like the admirals have been sitting around and doing nothing. In the manga, things were happening very quickly and simultaneously. For the most part, the admirals were busy with the higher squad captains and Whitebeard. Kizaru and Mihawk both made an attempt to stop Luffy's final charge but were blocked, and Garp's ineffectiveness is understandable. The only person I can't really defend is Sengoku. He should have done better.

This pace also makes it seem like Whitebeard has been standing still for long stretches . In the manga, he seemed most instrumental in holding up the admirals, engaging all 3 of them at different stages of the battle.