Why is it so rare for marines who know about how bad the Celestial Dragons to leave?

[Warmmedown]

The lower marines know pirates and marines both aren't perfect, but that the marines by and large make the world safer.

But what about Garp, Kizaru, Aokiji, Akainu, Sengoku and whoever else has seen Mary Geoise, God Valley and is privy to the plans of the Five Elders and other CDs. How come so few deserted after finding out the truth? Aokiji left, but it took ages.

It's not like there's nowhere safe to go either, considering Dragon made the Revolutionaries. And most of the marines don't seem to have family to worry about the well-being of.
Do they just hope to change things from within the system? Are Sengoku and Garp disgusted by the CDs?

 

[SSJ2]

Can the revolutionaries really be considered safe though? They've been attacked by Blackbeard and also the World Government. Maybe they consider the WG to be their safest option.

But yeah, perhaps they also try to apply their own justice system whenever possible, feeling as though they are able to make a more significant difference that way as opposed to going rogue.