In the prequels Qui-Gon states to Shmi that they take children in the Republic to identify them for Force-sensitivity, and it is also explicitly stated that having a spouse and kids would get you expelled from the Jedi Order; from there, we can easily infer that most Jedi and Sith don’t come from special lineages.
Most Jedi and Sith are not the PROTAGONISTS of the MAIN STORY like Luke and Anakin, that is the point of the “Rey Nobody” twist, and that is why most people expected her to be related to someone after TFA, because she was the protagonist.
Luke => Protagonist => Son of the main villain.
Anakin => Protagonist => Coinceived by the force
Both previous protagonist had an interesting origin
The point of the “Nobody twist” is that she CAN be the PROTAGONIST of the MAIN story being her own character and not being related to someone else and having a mundane background (“sold for drinking money”).
then TROS happened…