they have their own miners that is powerful and make it centralized in a way to steal coins from people, in a 51% attack like ethereum classic attack (the same way it happens on other coins blockchain too), you can have ethereum classic on you wallet but all gone because of the attack.
51% attacks cannot steal other people's coins. The most they could do is to prevent their transactions from confirming. Other than that, double spending requires the attacker to have the ability to create an alternative transaction which would require the private keys to create the transaction.
Did you mean the bitcoin blockchain? Bitcoin blockchian is safe from 51% attack. When the blockchain was not so strong like this before, no attack has occured on bitcoin blockchain before not to talk of when bitcoin blockchain is stronger and getting stronger.
It's not safe. The reason why Bitcoin isn't attacked is just simply a result of the cost-benefit involved. The cost is way too high for most to execute and for those that could, the profits gained from such attack wouldn't outweigh the cost. The blockchain didn't get stronger per se, it just makes such attacks less economical.