why is the bible considered an incomplete canon?

 Because it is unprovable And filtered not to contain all of its original books so is therefore incomplete

Your question begs the question:

By whom?

It's not. Go figure!

It’s not considered incomplete, unless you’re comparing what is considered canonical by Protestant standards against Orthodox or RCC standards 

The Bible itself says not everything is included in it.

Because it's not a monolithic work.  Because God know so much more than we do, including things we still need to learn.  Because the Bible is a collection made by man, not by God.  Because the books of the Bible mention other books of scripture that we don't have. 

And in spite of all of this, people still like to pretend that there cannot be any other scriptures nor revelation.

Only by those who need extra-biblical sources to get a following after themselves. That's why Jehovah's Witnesses have lots of publications saying what the right interpretation of the Bible is; that if you read the Bible alone, without their publications, you will go into darkness. That's why Joseph Smith Jr. had golden tablets of ancient Egyptian put into English, "The Book of Mormon". That's why the Gnostics had to depend on unbiblical writings to back up their unbiblical doctrines. On this question you are speaking for yourself, for the JWs, and for the Gnostics.

Some people don't heed divine warnings:

"I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book;

and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book." (Revelation 22:18-19). 

That moment was when Biblical prophecy and sacred scriptural submission ceased because they were complete. Any book or submission presented afterwards as 'canon' is a lie, even if it has managed to inspire millions of humans to action. That includes any 'new' religion, (like Islam), or any 'new revelation' supposedly related to Christ, (like Mormonism, JW's, etc...). 

The opinion that it is an incomplete canon comes from those that accept the apocrypha as part of the canon. The apocrypha was separated out from the Bible for specific problems with those texts. The problems span a whole spectrum of difficulties: authorship, dates of the writings, doctrines supported by canonical books, lack of reference to them from accepted canon, etc. etc. Some are even considered to have been written by spiritists. 

Bibles properly should have 73 books.  Catholicism is the One True Faith and it is the Catholic Church that compiled the first Bible.  Protestant Bibles are incomplete with only 66 books.