Think of a project to build a bridge across a massive river. Right at the beginning, you decide what the height should be. At that stage, it's just a line on the drawing. Easy to change.
Now roll on a year; you have scheduled all the steel deliveries, hired the workers, and the planning permissions, legal work and contracts are done. The foundations have been laid, and the project is on schedule. Then you realise that a supertanker travels down that river every week. The design is too low to let it under.
The cost of changing things now is horrendous. Much recriminations, emergency meetings, and discussion of temporary solutions ensue.
You have to pick a feasible option: the best is to make the bridge open and close. So when the supertanker goes through, the traffic on the bridge has to stop. The changes also result in the project being significantly over time and budget.
If you had known about the supertanker at the design stage, you could have made the bridge higher, to let it under without disrupting traffic.
This is exactly equivalent to the situation with information security. Designing it in from the start is cheaper, more feasible, and prevents infinite pain down the line. Requirements gathering needs to include information security.