Bitcoin has no CEO or formal central authority. But software development inevitably involves people who have more influence than ordinary users. Bitcoin core is extremely important because the software determines which blocks and transactions a node considers valid. Bitcoin.org itself says users running Bitcoin core independently enforce the rules. And the Bitcoin developer documentation explicitly acknowledges that Bitcoin's security depends on the behavior of software maintaining consensus.
Does it mean that developers control Bitcoin? If yes, How we call it as decentralized?
Developers have control over the Bitcoin network? Nah! But who are these developers? It is not a specific group of people, it is not an individual; anyone can be a developer as long as they have the technical knowledge, can code and contribute to the network. What this should make you understand is that, even among the developers, the powers is still not vested in the hands of one person that decides what should or what should not be done.
Again, the developers may have influence, but the powers to enforce any rule on their own is not given to them, they must reach a consensus first. Developers cannot impose a rule on anyone.
It is very beautiful how powers are shared in the bitcoin network; developers don't produce blocks and select transactions to include in them, that's the job of the miners.
@op, read more about decentralization, you will understand better.