This is explained by the fact that you have to remember what the main goal of (many of) the Sith was. It was eternal life.
That was what Plagueis, Sidious, and even Vader (who wanted to prevent Padme from dying) strived for, and Palpatine did achieve it to an extent.
As such, the ability for your consciousness to survive post-body-death as a Force Ghost would indeed be the most powerful thing that a Sith would think of.
“An infinite mystery is the Force.” Yoda lifted his head and turned his gaze out into the wheel of stars. “Much to learn, there still is.”
And you will have time to learn it.
“Infinite knowledge …” Yoda shook his head. “Infinite time, does that require.”
With my help, you can learn to join with the Force, yet retain consciousness. You can join your light to it forever. Perhaps, in time, even your physical self.
Yoda did not move. “Eternal life …”
The ultimate goal of the Sith, yet they can never achieve it; it comes only by the release of self, not the exaltation of self. It comes through compassion, not greed. Love is the answer to the darkness.
“Become one with the Force, yet influence still to have …” Yoda mused. “A power greater than all, it is.”
It cannot be granted; it can only be taught. It is yours to learn, if you wish it.
("Star Wars Episode III: Revenge of the Sith", novelization by Matthew Stover)
And, from a Sith viewpoint (From James Luceno's "Darthe Plagueis"):
... Had Plagueis unlocked the key to immortality, and survived after all? ...
The Muun might have lived another hundred years unchanged. He might have lived forever had he succeeded fully in his quest.
...
... and had devoted two standard decades to day-and-night experimentation with midi-chlorian manipulation and attempts to wrest a few last secrets from the Force, so that he — and presumably his human apprentice — might live forever.