So, with only a limited number of breeding males in existence, and a limited number of breedable offspring, it would be a lot easier to keep their population under control. Instead of chowing down on hundreds/thousands/millions of symbiotes in some sort of massive binging ritual, all they have to do is kill the specific offspring who are a danger to them. Which explains why it appears to be a special ceremony, and why no one else is aware of it.
That all makes sense. It's also possible that perhaps a queen (breeding sense) has to be trusted to withhold some knowledge from a system lord's offspring so he can have loyal minions instead of rivals, and perhaps some of the female system lords rose to power by fooling their powerful mates, making sure their offspring's loyalties were with the mother and not the father.
One of the best aspects of the whole Goa'uld storyline is the way they are a dysfunctional confederation of ruling dynasties, whose calculating power-grabs, betrayals, ahd shifting alliances could be the subject of its own body of literature, just like the royal dynasties in Earth's history. Shakespeare would've had a field day with them.
On a side-note, they showed the Goa'uld living in wild in lakes on the homeworld of the unas, but they never seemed to follow up on any of the implications of Goa'uld escaping back to the wild on other planets. Since they're small and aquatic it would be almost impossible to eliminate them if they decided to go to ground and hide, especially considering their long life spans. Perhaps the Goa'uld consider returning to such an existence far beneath them, though.