How did the Buddha become the Buddha?

Im curious how Gautama Buddha didn’t just become a sotapanna in his many past encounters with previous Buddhas? How can it so that many past Buddhas gave confirmation to the Bodhisattas future buddhahood, without the Bodhisatta being able to comprehend the Dhamma then and there?
Is it because the merit sowed in front of a Buddha together with the determination to become one naturally makes one develop the Paramis unknowningly without access to Dhamma in later births?
But what i don’t understand is how, when a bodhisatta meets a Buddha again, he doesn’t just ordain and finish suffering then and there.
I guess that’s what makes Buddhas so rare, because very few Bodhisattas end up prioritizing Buddhahood over Arahanthood or don’t accidentally become sotapannas in the course of many encounters with Dhamma.