If Sith are driven by total selfishness, then why would they care what happens after they're dead? Yeah the more we delve into it, the less sense it makes. An occupational hazard when dealing with all that Force jazz.
The only thing a sith cares about more then themselves is hating the Jedi. Therefore, you'd want an apprentice to carry on that hate so it won't be forgotten after you're dead and gone.
If their hatred of the Jedi exceeds their selfishness, and assuming they have any brains and strategic ability at all, then why don't they squelch their selfishness long enough to assemble into groups and thereby be more effective in fighting the Jedi?
The Sith philosophy makes sense only if selfishness is at the top of their value system, and supercedes all other considerations, including effectiveness. They allow selfishness to deliberately inhibit their ability to construct a functional system that could make them more effective in every way.
Power is not a zero-sum game. A Sith could collect a gang of, say, 12 Sith, share power equally, and end up with more power than he could get on his own, even if it were only 1/12 of the total power that the group has.
All of human history is based on the idea that by organizing themselves into groups, humans can get more goodies - safety, power, money, food, etc - than they could individually, even if they have to share with the group. If this were not true, we would still be living in tribes of about a dozen people, and there would be no cities, nations, organizations, corporations, etc. Our only basis for organization would be genetic - to help those genetically similar enough to us that it made sense to sacrifice to pass on the genes that we share with them.
The ground rules are not different for the Sith, so why are they going against the very nature of a social species? And regardless of what species they come from, we know they are all social species, because they couldn't have space travel otherwise. If there is a truly non-social intelligent species depicted in the Star Wars universe, I haven't run across in (in movies or TV).