Blockchain is considered to be secure for several reasons. First, the decentralized nature of a blockchain means that it is not controlled by a single entity, which makes it more resistant to tampering and censorship. Additionally, the data on a blockchain is encrypted and stored in blocks that are linked together using complex cryptographic principles, which makes it difficult for attackers to alter or delete the data without being detected. Finally, the consensus mechanism used by most blockchains, which allows multiple users to verify and approve transactions, adds another layer of security to the system. Together, these factors make blockchain a highly secure technology.