Why can't they put the emperor in a dreadnought?

...

Dreads are designed for physically destroyed marines, not psychically destroyed gods.

They did, its just his is a giant throne that guides spaceships across lightyears through the warp, rather then shoot xenos.

Because they literally use his ass to plug chaos from entering reality.

Not canon anymore.

Because he's too busy being a battery and focusing the galactic lighthouse.

Considering the size of the machines keeping him alive, it would more of an Imperator Battle Titan

Because they need him where he is. The golden throne is the only life support system advanced enough to keep him in check, and the infrastructure surrounding it is unparalleled. The golden throne is directly hooked up to the choir of the astronomicon, and the big pit where psykers are sacrificed to him for food. Hooking a dreadnought up to those would be near impossible, especially if you want him to stay mobile. In addition to that, even if he could physically move, he wouldn't have the mental strength to. 100% of his attention is dedicated to preserving the astronomicon, if that were to fail the imperium would simply crumble. He can actually psychically communicate with people, but it's pretty much never worth it to shift 0.1% of his attention away from the astronomicon.

TL;DR:

the golden throne has machinery that goes up and down the entire imperial palace, and often deep underground

it wouldnt be a titan so much as a giant spaceship
you would also need to unplug the golden throne to put him in the second one, and if someone misplaces a single solder you might kill the emperor in the process

Really? Is it just a life support now?