Asking why Kal-Sarpa yoga were not mentioned in ancient texts is like asking why Trolls
were not a subject of Leonado da Vinci's paintings! Well, in a way, they were - the famous “Grotesque” series is the proof!
Rahu and Ketu are just two 'imaginary' points - they are not physical bodies, but they were given as much importance as other heavenly bodies in ancient texts- is this not proof enough!
Yes Kal-Sarpa yoga - specially the partial ones may be found in good percentages. Who says a person with this placement will not at all be famous/rich or both? but most struggle and can't succeed without solid supporting placements and dash [periods] of those planets.
The Panch Maha-purusha yoga [five great native placements] and other Maha-yoga [ruchak,shash, malavya, raj-hamsa etc.] are also found in very high percentage, so does Mangalik Dosha but they also affect the native to different degrees- depending on lot many other factors. This is not uncommon!
This is the reason that the relative and absolute strengths of planets, aspects, cycles (and sub-sub-sub-sub cycles), transits, AshTak-Varga Charts, Divisional charts are also studied in great details.