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.