The internet is decentralized.  Who defends a website from hackers?

Each user is responsible for their own wallet and for protecting their own wallet from hackers.  If you don't feel capable of that, then there are services that will offer to protect the wallet for you (such as Coinbase.com) in which case you are trusting them to do so.

There are thousands of copies of the blockchain all over the world.  In order to "hack" that, the hacker would have to do one of the following:

• break mathematics
• simultaneously hack EVERY node in the world on all operating systems including copies that are not online
• Find a significant weakness in the SHA256 hashing algorithm without anyone else in the world being aware of the weakness

Of those options, only the third one is ever likely to happen, and it is far more likely that the weakness will be discovered by a researcher before a hacker figures it out.