Serious question for Christians: If Jesus prayed that His followers "may all be one" (John 17), why are there thousands of denominations today?

https://archive.org/details/the-divide-of-christendom

I linked my thoughts on this. The link leads to Internet Archive. It's lengthly, so here are some spark notes.

I think people have different fundamental principles, or assumptions, that initially drive people in different directions using mostly the same logic. Some of the principles are what love is, what our purpose is, how free will works, and others. Those assumptions are often overlooked or left unknown. These principles lead to the major differences in opinion that make up many major denominations we see today.

I did some research on the different major divides, like the Great Schism, and included some contextual issues at the times of division.

Then, I go into the practical life of a Christian and what is really expected from us.

I end off with some practical take aways and a push for major church reconciliation.

Throughout the discussion, I try to stay grounded in Scripture.

I'd really appreciate constructive feedback.