Blockchains handle complex operations and a significant number of users. They must be tamper-proof to prevent fund or critical information theft, crypto asset destruction, or double-spending. 

Here is what makes them secure.

Blockchain operations are governed through sophisticated mathematical models and innovative cryptography rules (e.g., hashes that link separate blocks and make them immutable), which are extremely difficult to manipulate.

Storing a blockchain copy on each computer in the distributed network means no single node can exploit it. 

To do that, one must overtake 51% of the network, which requires enormous power and makes such attacks pointless.

Nodes maintain the network’s security by monitoring and verifying the legitimacy of transactions.

They are incentivized to act with integrity through block rewards, transaction fees, or governance rights.

Blockchains are fully auditable, transparent structures. The details surrounding each transaction are publicly available and easily accessible.

As a result, hackers and stolen funds can be tracked and located.

Blockchains are Byzantine Fault-Tolerance systems. A BFT system can continue operating even if some nodes fail or act maliciously.

The blockchain won't be affected even if hackers compromise or corrupt a node.