The protagonist didn't take over someone else's body; he was reincarnated, and his memories awakened when he was nearly six. As for why, here's a spoiler: it's a non-system "cheat" ability, and I'll explain it in the middle of the story.
Neither of the protagonist's parents was a particularly powerful Soul Master. They were only at the Soul Elder or Soul Ancestor level, so their Martial Souls weren't necessarily very powerful.
The higher one's Innate Soul Power, the stronger one's Martial Soul. But a powerful Martial Soul doesn't necessarily come with high Innate Soul Power. Even a Martial Soul as powerful as the Clear Sky Hammer didn't always come with Innate Full Soul Power; some people had only three or five levels. Huo Yuhao in Douluo Dalu II also had very low Innate Soul Power.
The protagonist had Twin Martial Souls because he cultivated the complementary Supreme Universe Art and Five Thunders Heart Art. Later on, his Martial Souls would gradually grow as he improved those two techniques.
As for why the protagonist had only five levels of Innate Soul Power despite cultivating for two months before awakening his Martial Soul, it wasn't an arbitrary weakening. Martial arts required a period of laying a foundation. Both The Degenerates and Tang San in Douluo went through this process.
Tang San began laying his foundation and cultivating when he was only one or two years old. The original story mentioned that he didn't break through the first level of the Mysterious Heaven Skill and attain Innate Full Soul Power until three months before his Martial Soul Awakening.
As for Shrek's members all having eight, nine, or ten levels of Innate Soul Power, the original story mentioned this too. At the beginning, Tang San learned that Xiao Wu had Innate Full Soul Power. That was because their natural talent and Martial Souls were powerful enough to produce Soul Power automatically upon awakening. Tang San, on the other hand, had cultivated his Soul Power step by step.
The protagonist couldn't possibly awaken his Martial Souls and immediately attain Innate Full Soul Power just because he had Twin Martial Souls.
Having Twin Martial Souls didn't necessarily mean someone had Innate Full Soul Power. Innate Full Soul Power could occur when both parents had Martial Souls of similar quality. Even Martial Souls that were somewhat weaker could very rarely produce Twin Martial Souls if their quality was similar. They didn't have to be on the level of the Clear Sky Hammer, Blue Silver Emperor, Death Spider Emperor, or Soul-Eater Spider Emperor.
There was one more important point: the original story mentioned that Tang San had considered teaching Xiao Wu the Mysterious Heaven Skill, but by then, her Soul Power was already in the dozens. She would have had to start cultivating the skill from scratch, making it more trouble than it was worth.
In other words, even if Oscar had started cultivating the Mysterious Heaven Skill too late, before awakening his Martial Soul, he still wouldn't have awakened with Innate Full Soul Power.
Before you continue