The first symbiotic event was a million times harder than the 2nd or the 3rd. The first time the host had the extremely hard task of dealing with any DNA and RNA produced by the guest during it's life and death. The host had to evolve stuff like a cell nucleaus and sex to live through it and alternative splicing to deal with the fact that all it's genes were damaged by selfish genetic elements that came from the guest. Integrating any later symbionts is still hard, but not nearly as hard.

It's possible that the first symbiosis that let to the origin of the eukaryotes is not a one in a billion years event, but one in a trillion or one in 10^20 years or ever rarer. That is it may be that in a billion planets with simple life forms only one "creates" complex life like animals. It can be the great filter that leads to the Fermi paradox.