Because if it is not - it is simply a centralized database of payments. Like an SQL table of transactions. This has been a solved problem for decades. Visa and MC processing servers all use cryptography in many parts of the stack, and yet no one calls Visa a cryptocurrency because of that. This is why if you see anyone use the word "cryptocurrency" when in reality they talk about a legacy centralized (thus lacking all decentralizations perks) database - you know they are assuming you have no actual knowledge about the area and are trying to buzzword you.

Forget about centralized/decentralized. There is an ecosystem around existing cryptocurrencies. Wallets, exchanges, dapps, etc. All of it wouldn't work use they an SQL table of transactions.

I'm not sure that something like Vision Plus is very similar to a Blockchain.

Aside from both using cryptography, in the sense, of encryption.

So... if such buzz words are used, it seems like a huge lie.

The problem with centralisation is that it enables a kind of corruption: entities can engage in both financial censorship and arbitrary money printing.

That being said a cryptocurrency does not have to be decentralised.

However it is very important to understand the difference.

Today even decentralised cyptocurrencies depend on centralised-exchanges, and the consequence is that despite the constraints built into a blockchain algorithm are able to sell unlimited amount of the crypto to users without actually being in possession of the algorithm-limited crypto.