>Can a star destroyer enter a planet atmosphere?
Maxim 11: Everything is air-droppable at least once.
Now, whether it's capable of slowing down and getting back out in one piece…
>I don't like the idea that they only come from one planet but I do think they should be very rare, which supports the lore that darksiders mostly use synthetic ones.
Either that, or the Jedi were only using one source out of tradition/snobbery.
There could be much more, but… Most of the "Sith" are either:
A. Former Jedi who went full angry teen and willing go with any Anarchist Cookbook grade dangerous crap. Or
B. Backwater prodigies, who learned basics from a found holocron with someone's crazy rants and trained themselves.
Neither of which is likely to give comprehensive knowledge of this matter, so they didn't really choose to do it as much as used the first accessible option they found.