Because men like to complicate simple things.
The Lord Jesus Christ, Who is the Head of the REAL Church did not come to divide belivers, but to unite them:
“I do not pray for these alone, but also for those who will believe in Me through their word,
that they all may be one, as You, Father, are in Me, and I in You; that they also may be one in Us, that the world may believe that You sent Me.
John 17:20-21
Men are messy. And God is SOOOOOOOO patient.
List of Christian denominations:
Wikipedia
https://en.wikipedia.org/wiki/List_of_Christian_denominations