Git's internal data structure is a tree of data objects, wherein each objects only points to its predecessor. Each data block is hashed. Modifying (bit error or attack) an intermediate block will be noticed when the saved hash and the actual hash deviate.
How is this concept different from block chain?
Git is not listed as an example of block chains, but at least in summaries, both data structure descriptions look alike: data block, single direction reverse linking, hashes, ...).
So where is the difference, that Git isn't called a block chain?
The question reads: Why is Git not considered a “block chain”? So this is asserting that there is a wide-spread opinion that Git is not a blockchain (an assertion that is illustrated and corroborated by the answers preceding mine on this page) and asking for the reason of the prevalence of this opinion. This is a good question.
Taking the question literally, the answer could be that the blockchain term and concept gained popularity as part of the digital currency operation called “Bitcoin”, and hence came to be associated with how Bitcoin does things: which is by using a lot of computing power to calculate a specific hash including a nonce to meet certain arbitrary requirements, which is by allegedly having no central authority, which is by being “independent”, maybe even “democratic”, and the rest of the kool aid; and as these things are not seen in Git, well, Git cannot be a blockchain, right? And so the question would be answered literally.
Hidden behind this prima facie question is another question: What is a block chain? Now you could look up a definition somehwere and copy it over here, but I didn't do that as I have made up my mind years ago, when listening to a podcast about Bitcoin that strove to explain the new concept of a blockchain, that a blockchain works like Git and I don't intend to let my precious understanding be misled by random claims on the internet.
So what is a blockchain? What's in the word?
Nothing in the term “blockchain” presupposes the requirement to include a nonce in the content so as to come up with a hash of so and so many leading zeros. (This requirement is only there to be able to control the blockchain by computing power and so, ultimately, by money.)
Nothing in the term “blockchain” presupposes the existence of a network, let alone a decentralized one.
Nothing in the term “blockchain” presupposes any “independence” from “central authority”.
The term “block chain” only presupposes blocks (of data) chained together. Now what is a chain? Is it just a link? No, it is a strong link designed to hold things together by force.
A simple linked list doesn't qualify as a blockchain because the contents of the chunks of data in the list could be altered while the list would continue to link back and forth just fine. This is not how a chain works.
To make a link of blocks of data into a chain of blocks of data, the contents of the blocks need to be checksummed (digested) in one way or another and this checksum (digest) must be part of the link, making it a strong link protecting the content, preventing it from being altered. This is a blockchain.
And this is what Git does, and hence Git is a blockchain, or works as one, if you prefer.
To close the circle, let's ask again: Why is Git not considered a “block chain”? It could be because many people, perhaps even a large majority, do not focus on the essence of a concept but on blinking accidents.