Crediting the origin of the idea is the whole point of citing sources. Learning something from someone doesn't mean the idea is yours now. It means that when you repeat that idea, you should cite the original source of the idea.

This is just how scholarship works. It's not needed in the kind of day to day most of us do, but when you're writing a thesis for a PhD, this stuff matters. You're making the argument that you're expanding the totality of human knowledge with your dissertation, and that requires strict source citing to separate your original scholarship from the sources that influenced it.