I've been asked a lot of times, especially when I was working for an antivirus producer, why can't we simply write a software that always protects the users.
Well, there is a short answer and a long answer.
Because 100% security does not exist and because most people are hackable due to being ignorant on what security is (of course, until he/she is hacked first time, and sometimes not even after such an event).
Long answer, which I massively shortened by not touching all areas and not going into details:
The reason is the ignorance about everything that might happen but it is not certain that it will happen. I mean, would anyone close an insurance if it would have not been required by law or be afraid of the consequences?
- a lack of knowledge, understanding, or education : the state of being ignorant[noncount]
- ignorance is bliss
— used to say that a person who does not know about a problem does not worry about it