Why is copy and pasting code from e.g. Stackoverflow not considered plagiarism?

Comments

[Nicholas Stimpson]

Code from StackOverflow is licenced for use CC BY-SA, either 2.5, 3.0 or 4.0, depending on when it was posted. You should comply with this.

[Rick Delpo]

Because most code is open source which is sharable and we all live by copy and paste anyway. It would be quite difficult to copy an entire program. We just use bits and pieces at our convenience. Also Github shares much more than Stackoverflow. This is all just my humble opinion.