Why do programmers choose to write their own code instead of using code that is already available online?

Read also:

• Why do people keep saying they have evidence and have presented it that proves you're wrong even though they have none and haven't presented anything? Furthermore, what do they think you're wrong about?
• Would you let your partner cheat on you every now and again?
• Does any unofficial Roman Catholic card exist in the world to play? In Italy, Rome & the Vatican City, can practising Catholics get any discounts as a tourist, & / or privileged admission to certain sights, with different rules for non Catholics?
• Can the belief of not worshipping Christ held by Jehovah's Witnesses be disproven using scripture alone?
• How can I help my cat adjust to sleeping in its own room after allowing it to sleep with us as a kitten?
• Can you fly an American flag in the UK in your own private property there? What is the UK’s government stance on that? And if yes, do you also have to fly the UK flag or the American flag can fly solo?
• What should I do if I love someone who does not love me?
• Has anyone who has been a victim of a narcissist made contact with the other victims of the narcissist? Did it help to confirm what you suspected about the narcissists?
• What is your review of "Regent", episode 5 of Season 2 House of the Dragon?
• How can Democrats not feel hypocritical when they urge Trump not to be vengeful should be become president when the Democrats are trying to put Trump into prison?