I have been reading about blockchain for some time. The basic premise of security of the blockchain, as far as I can tell, is that in a very large network, change in the hash value of one block renders every next block as invalid. Moreover, one also has to outperform other peers. Since the hash value is sensitive to the data inside, it is very difficult to hack.
However, what prevents me from just stealing the data. Let me reemphasize this: I am just copying the data from the blockchain. As far as my understanding goes, the hash value shouldn't change because the data is still the same.
I am not able to understand how does the blockchain prevents this. I am sure there is something I am missing because otherwise there is no way Bitcoin (or any other crypto-currency in that regard) provides 'safe' and 'anonymous' transactions when the full chain is available to everybody and hash value doesn't change on copying the data. I mean this is practically the best place to steal data.
So, I will repeat the question: how does the blockchain prevent just theft of data?