I know why. It is a very tightly kept secret.

you know, just incase

Y

*yep

like just in case he is on his last life and can't respawn

Yep. Did you know that Ollanuius Pius was also a perpetual, and served in the battle of Verdun?

fairly certian 1 third of the named characters are perpetual

*certain

... Having lived through all of recorded human history, he makes first-hand reference to historical and mythological events such as the being one of the Argonauts, participating in the Battle of Austerlitz (during the Napoleonic Wars), the Siege of Verdun (during the First World War), and the Battle of 73 Easting (during the Gulf War), amongst others.

https://wh40k.lexicanum.com/wiki/Ollanius_Pius
[source]

I know

Or if they told the orks about the emperor and how strong he is the orks will all want to fight him therefore he will regain his body so they can fight him!