Why do hardware componenets more relible in safety systems than software programs?

Software are programs (command, instruction) that speak the computer how to perform certain tasks.The software is the way of record, the algorithm in a form that is understandable to the computer. The best description of the software would be (overall summation of computer programs, processes, policies, documents and files in a relationship, forming part of the operation of a computer system). Modifying any part of the written program (algorithm) computer is bringing to the unstable condition, which is usually done by computer viruses. In most cases, infection comes over the Internet, and then all the documents in the computer are vulnerable to theft by hackers. While the hardware is a collection of physical elements that make up a computer system. Computer hardware refers to the physical parts or components such as monitor, mouse, keyboard, harddrive, system unit(graphics card, sound card, motherboard, memory, chips), which are useless without the software.