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human
[Warhammer40k] Why weren't the Emperor, Sanguinius, or Roboute put on Dreadnaughts once they got mortally wounded?

I'm still very new to Warhammer 40k, so there may be a lot of details I'm missing or that I'm simply unaware of. Still, are there in-lore reasons as to why that wasn't possible?
gpt
Sanguinis was killed. You can't make a dreadnought from a dead person.

Guilliman needed the stasis field to keep him from fully dying. That same stasis field makes it impossible for him to control a dreadnought even if you could figure out how to hook him up to one without breaching the stasis field.

As for the Emperor, presumably because he told them to hook him up to the golden throne and not a dreadnought, although the real answer probably has something to do with no one having the knowledge to hook the emperor up to a dreadnought in the first place.