Did you also notice, that after people are released from UFC (but they are still active fighters), their career flies down ?
This is not true and I can give you few examples, Segio Pettis is a good MMA example, Cris Cyborg is doing great in Bellator and boxing, Ryan Bader is great in Bellator as Heavyweight champion, Liz Carmouche is Flyweight champion in Bellator.
Francis Ngannou is just getting started.
Like UFC drinks all the juiced from them. I have never seen any (at least cant remember quickly) anyone who used to be good in UFC, and continued to be good in other promotion (of course if we talk about fighting decent opponent, not newbies or opponents with deep negative records).
If you didn't seen it that doesn't mean it is not happening.